I think it comes down to organization? Having a writing software or something with all the important cast members, their development, their involvement in the plot, their basic bio might be a good way to keep track of it all.

I have a whole bunch of characters. I have bios made for them, I have short descriptions, I have index documents with basic info to keep it all straight when writing.

The more experienced I've become, the smaller the core cast of things I make tends to have gotten. In fact the main reason Errant has what is a comparatively large cast for my work is because it's a reboot of an older comic, and even then some characters are less relevant than they were in the original (Zeph, the pink-haired punk guy is a definite example, he has a much smaller part in Errant than he had in the old comic, Fan Dan Go).

These days I tend to take an approach of "never create a new character if an existing one can plausibly fill that story role". I never want to do what Hussie did with Homestuck and make a cast of incredibly compelling characters all in one story but then fail to satisfyingly wrap up all their storyarcs. Better to save a character concept I can't find a place for to use in another story!

I also think even if you are naturally good at remembering every detail of your characters without even needing a database, it's best to keep the cast tight and not expect readers to remember every tiny detail and who every minor person in the story is. I may eat, sleep and breathe Errant, but my readers only get a couple of pages a week with several days in between. A lot of readers who follow Errant week-to-week rather than having recently read through had already forgotten who Sarin Aoki was in the almost 2 years between the prologue and her appearance in chapter 4, in spite of her having been mentioned a few times between and being really important in the prologue. That actually surprised me and taught me a lesson of not taking reader attention for granted; it's never as focused as you hope!

There are always a few readers who remember literally every little detail about characters, but it pays to assume the majority of readers will not remember supporting cast, so to never lean anything important on recalling specific things about them.

I tend to have like having large casts in my stories maybe because I grew up with stuff like the Simpsons.

I guess for me, I usually divide the characters into categories. There is the main characters who get the most attention. Then there are secondary who get some attention. Then there are supports who have some purpose in the story but aren’t going to get that deep backstory. Then there are bg characters simply to fill space, they might have one or two lines, sometimes a parent of a character will fall into this, like you know someone’s mom exists but she isn’t going to get any development.

The short answer, my lore is what helps me keep track. Thanks to that, I know how each character connects to the other or what kind of impact they might have later in the story.

Casts just kinda... bloom outwards for me. Depending on the genre and story, characters may not really outsiders. Three people locked in a murder cabin might not even mention other people the whole story.

But how it goes for me tends to be-
-Start with a core cast, usually 2-6 really central characters
-Oh, I need the characters they need to exist. If the main is close to her younger sister, I want her to feel like more than 'generic little sister'. If the main is in constant conflict with his academic rival, I want the rival to feel like an interesting foil with his own motivations and life.
-Now there's roles I need to fill. If my character's a spy, who are their informants? If I want to have a wizard tower, who runs it? If I have a political story, who leads the various factions?
-Finally, the things that fill out the world. If you want to tell a high school story, just giving the teachers a spark or personality can do wonders, or giving an adult character a few interesting coworkers.

It's not like each one of those characters needs their own arc. Often their role is to advance somebody else's arc or provide conflict. For me, it's thinking them through just enough that they feel like more than their story function and I can say 'I know who this person is' well enough to put them in a scene. As somebody who writes a lot of mystery and intrigue, having a filled out background cast gives me a lot more tools in my toolbox.

As long as I stay on project, tracking it isn't too hard. I need notes for things like ages and names, but not for the more conceptual stuff. If I leave something for a year or two... better hope my past self kept some good notes.

I feel like having a large cast is easier if they all have specific, clearly defined roles. Like, you have an idea of how important each character is and how much 'screentime' they will get, and that stays constant through out the story.

In one story I wrote (which was admittedly the most complicated novel I ever attempted) there were two main characters, each of which had their own rather large accompanying cast. ^^;

On one side we had Character A, who was captured by B, started to fall in love with C, befriended D, and had a rivalry with E. Then she escaped BCDE (who were all part of the same group of 'antagonists') and sought help from her old friends F and G. After that failed, and she was re-captured by H, she had to return to the antagonists, and eventually met I, who became her most hated enemy.

And on the OTHER side, we had Character J, who teamed up with K and L to solve the mystery of A. She met M, who betrayed her, and was attacked by N, who killed M but let her go free. After being antagonized by O, she teamed up with P, who was a friend of O's and kinda-sorta betrayed her too, in the end.

All these letters represent named characters with fleshed-out personalities: the reader is given enough information to at least be able to predict how each one will speak and act, so they aren't just background nobodies or throw-away characters. Each has something to do with the main plot and a role to play in it (and honestly, that isn't even all of them).

AND YET, of all the things I had trouble with while writing that story, keeping the characters straight wasn't really one of them. ^^ Because each new character acted as an accessory to A and J's journeys. They were introduced and fleshed out one at a time in accordance with the story beats they were responsible for, and if they were spotlighted, they were spotlighted WITH their anchor characters, not instead of them.

And apparently this is important, because when I fail to make sure that's happening, things quickly fall apart. Case in point: yesterday I was looking through an old comic script of mine that also had a large cast, and it soon became apparent that I barely remembered any of the characters' roles in that story. Like, I was genuinely confused while re-reading my own work. ^^;

And now that I think about it, it was probably because the magnitude of a character's role basically changed according to my whims. Everyone did whatever they could towards resolving the plot whenever they wanted, and because the older side characters tended to have more power and influence than the 13-year-old main cast, they gradually took over the whole story.

One guy in particular, who was more of a minor antagonist in the beginning, was a full-on main character by the end, and while I think his character development was beautiful and well done in isolation...it also pushed the ACTUAL main characters straight out of the plot. Like, they were literally sitting at home while all the other characters were making progress together.
I'm not surprised that I quit working on the script soon after that part...although, now that I've learned my lesson, I kinda want to try it again. :9

I'm used to it. I have this writing ideology that when you're creating a character you're creating a mini universe within this fictional universe; meaning you might as well flesh them out as much as possible. This type of thinking helped me keep up with all sorts of characters.

Another thing I do is whenever I make a character, it's an opportunity to expand the world from another angle; so I'm like "Okay, I'm expanding the lore of this village through this character. It's cool that I'm returning to this character since I can't really do that with OTHER characters".

One last thing: I try not to give anyone the role of protagonist so I don't have to wrap my head around this one character. The world is one singular character and the characters themselves are organs with a function.

Also OCD is a mf.

It can become annoying when there are so many characters I have to page back through the story to figure out "Who is that guy? I know he appeared before now but..." I've seen some scifi authors write their big, fat novels with huge cast of characters by putting a Dramatis Personae page in the front pages with names & short explanations.
It seems to me, looking back on it, that for such "well populated" stories I don't become as engaged or in sympathy with the characters as I do when there's a smaller number & maybe just two or three who figure prominently.

My first comic had a cast of 17 students, 4 teachers, 4 parents, 17 people to keep track of as a ruling entity doing court politics, 8 people who were just an outcast group, 6 talking bird creatures that sure did exist for some reason, and 6 godlike entities that controlled the universe. Anyway it's needless to say I only wrote 40ish pages of it before I had to stop. Maybe one day I'll do it justice, but only by doing a big cull of the entire cast. Like it's totally fine for them to exist in my head, but it's just way too hard to bounce between all those people without it getting confusing in a comic. In a book, it'd probably be fine. In a comic, it is hell because I'm the only one writing it. My Hero Academia and Marvel and DC make it work (barely) because they have teams of people to make issues really quickly. I'm the only one making my own work though so I just have to keep it simple.

Like my current cast is 3 people mostly, and 2 that will be recurring occasionally and it's so much easier for me to do the pacing. Especially since my comic only updates 2x a month. I have plans to introduce more characters, but only doing it slowly and one character at a time so that way I can do them justice and help readers connect with them.

The size of a cast doesn't matter as much as its quality, however, some longer and more complex stories may require a larger cast.

As a general rule it's better to introduce characters gradually and as they become relevant (one could foreshadow some of them with details like family pictures, mentioning them to tease their relevance or even some background cameos)

If you introduce them way too early and/or consecutively, readers will lose track of them, specially if they aren't relevant at that stage of the story.

I do have a large cast of characters, and what I try to do is treat them in the most reasonable way possible in terms of priority, there are different types of priorities in my case, I don't know if everyone thinks the same as me.
1- The Incoming Deaths
These ones are the most important for me, even more than the main character, if they are going to die soon, they need to be relevant somehow, leave their mark, be in the spotlight, I want people to know them before they die. I want to do this with this particular character because I don't want to shove all the backstory and development in one chapter just to kill this character at the end of the chapter, I think that is a cheap way to create empathy but at the end, the character doesn't matter because it never mentioned again or people don't remember them and I don't like that. I think they need their time.

2- The Setups
For stories that have multiple conflicts and involve different characters, I want to give the spotlight to these ones for a moment, even if they are not an active part of the main plot. For a better explanation, I'm not saying that these characters are just random characters that are on the other side of the planet or something like that. These characters are in the same place where the main plot is happening but they are not a big part of it, they are not relevant right now so people might think that they are just secondary characters or side characters that won't appear ever again. I think is a solid way to introduce future plots in the actual story, because when people introduce new characters and try to point out that they were part of the main story but we never saw them anywhere before. it is a big potential that bad retcons happen, especially when they try to justify their relationship with the main character.

3- The Main Characters

4- The Secondary Characters

5- The Witnesses
These ones are almost like background characters, they don't intervene much in the actual plot, but they can be important in future plots in different ways. They could be the ones who told to the bad guy about the MC skills and actions, they could defend the MC in a court for their actions, the possibilities can be endless.