WAAAAAAAAAAY too many characters; basically a comic universe of my own.
I feel like I have this issue.
I think it’s worth making a ranking system of characters who will get more focus and development vs those who are just there to fit a role (ex. shopkeeper or family member).
I think it’s also helpful to combine similar characters into one. Or think about if a preexisting character can fill the role. And also being ok with characters who might only appear temporarily or for one story plot but will not appear later on.
I actually have planned EVERY character the story needs. I can't add more, except background characters obviously.
My story it's already longer and more convoluted that I wanted it to be. So, yes, I HAVE to resist the urge of adding a new character to the mix. At least, I'll try harder for the next chapters...
Lol, yes, but nah. I had planned out my whole story (with room for improv) and "sectioned" out bits for certain chars to be featured - like have only these 10 appear in one "season" and some be recurring- and some to appear once and never show up again. Ironically given what I've published so far it doesn't feel like I have that many, but the longer the story goes on the bigger the roster grows.
I have maybe 50 or more characters in total (not including future improvs) yeesh! But I tell my story slowly and as fully as possible so each character feels important to the story.
THIS… In the story I’m currently writing, sometimes I feel like there’s not enough characters. The problem is that most of the characters are members of quite a big organization, so there needs to be a lot of them. However, I just can’t sideline any characters. I just feel like that would be unfair to those who like this character the most (it happened to me way too often, I would have a favorite character and then they would just 
die without any backstory. I don’t want that to happen to anyone else). So even though I don’t have enough characters for a historically accurate setting, I still feel like it’s too many of them 
anything I can do?
Nice, sounds like you are able to find a good rhythm to introduce all your characters ^^ I definitively don't think it's a problem to have 50+ characters if you can pace your story well!
My comics and stories tend to have very small casts, so in each individual story? Heaven's no. Besides maybe the series my friend and I worked on privately.
But... if we're talking about characters in general, with their own personalities and history regardless if I ever end up using them for something, I got a ton. I've never counted them all, but my private works with both my friends I have about 50 characters each, not to mention all the mini private projects, and the solo ideas I have... hundreds of ocs, man.
So I suppose my answer as to how to deal with wanting to make more characters is: just do it. They don't need to be part of your current project. They never need to be in projects that came to fruition bar silly doodles on a social media account-- or a spin off as you said. Side characters added willy nilly will bloat and bog down the story, you can set them in the same universe by all means, but don't put the integrity of your story on the line for it. I'm sure the world is huge, they dont need a role in the current story.
I've been struggling with this too. I'll make a base cast of characters, then realize: "Isn't it weird that everyone is an only kid? What about the other people in their life like parents, cousins, friends, colleagues etc.?" I don't know if it's worth establishing these side characters, if they're not that important to the main story.
I make characters to advance story beats in my comics. I don't have characters to just have characters. If they don't advance the story, there is no reason for them to be in the story.
I love designing characters, but I’m the same as @BoomerZ I only ever design new characters when the plot calls for it. Otherwise I won’t end up caring about them.
I have so many characters, send help
Serious answer, I've written a lot of stories over the years, some of which end up with a huge amount of characters just from the type of story it is (e.g. stories with expansive worldbuilding). Most of them only really exist within the context of the scene that I'm writing and never have reason to interact with each other, so I don't find it overwhelming or anything... unless I sit down to think about how many characters I have, haha.
In the context of worrying if you have too many characters, the main thing I'd ask yourself is if the new character(s) you're adding add anything to the story. If the answer is "no," then maybe you shouldn't include them. But so long as you introduce each character sufficiently and don't just throw a bunch of new characters at your audience at once, you can really get away with as many characters as you want.
My comic has 6 main characters. I try to give them roughly the same amount of screen time. I make sure all 6 appear in some form in every chapter, and each character gets a story focused on them per volume. I think I'm balancing the 6 well enough, giving them all time to show off their personalities and develop.
My supporting characters are all pretty much throwaway/interchangeable. I wish I could make them/the world more fleshed out, but I only have a limited amount of pages per chapter, so I kind of have to sacrifice fleshing out anyone outside my main 6 to make sure they're all consistently written.
I'd try to focus on only the characters vital to the story currently being told. If a new character is interesting enough to you, try figuring a way you can naturally connect them to your comics story. If they're interesting/strong enough, I think you'll find a way to bring them in.
That´s one of the things I´m working on to become a (better) writer.
One example. I just wrote a story and I started with 2 characters standing
in line for a job interview. Then of the characters saw another character (character 3 already)
and so character 1 told character 2 a story about character 3. In the story there was
another character (4).
I added a narrator (1) and a listener (2) to the story to tell the story of 2 other
characters (3 & 4) and ended up with an overcomplicated structure, 2 locations
which have nothing to do with each other and a story which doesn´t help the
cartoon style comic that I´m drawing.
Solution for me, getting rid of character 1 & 2 and just focus on one location and
2 characters and that made the story easier to understand.
I can´t say anything general about writing or the stories of other people because
I don´t have the knowledge or experience to give advice and stories are individual.
I enjoy watching a series like Lost with many characters, story and backstory,
in Game of thrones it was the opposite for me. When I read comics I prefer limited
characters and locations because it makes it harder to follow and I often wish comic
creators would write / draw stories which are easier to understand because I often
don´t have a clue what´s going in the story because it´s too much information
It's probably the opposite for me! I always feel like I should make more character cuz (for example) why do you only have 1 friend in a whole school?? haha! I never bother with character designs if they don't reappear much throughout the story.
It's a problem I probably need to fix because I can't keep making faceless NPCs! 
NONSENSE! No such thing as too many. xD
For stories, I tend to lock it in airtight for the cast and what is being told and who is important to tell what within it.
As for random OCs, DnD characters, and undeveloped stories, I just leave them all floating around. Maybe draw them. Maybe develop the story and see if it's anything worthwhile. Otherwise, just have fun with it. :}
Also, it's fun to share them for Art Fight. So that's something you can do with them. ;D
Oh, I definitively love sharing my OCs on art fight! Then I start writing more on their profiles. Then I develop their traits and backstories more and more... then I want to feature them on my stories because I've gotten attached to them xD
Almost all OCs I end up making are part of a world/universe that I'm already writing, even if they're not directly related to the plot (like the younger sister of a secondary character...). I guess I just gotta see if it's worth to put them on the existing main story or maybe make a side story lol
