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