How to Write and Maintain a Large Cast

I’m not entirely sure if this is the right space to post this, but here we go!

When I first started writing my series in 2019, my cast of main characters quickly grew bigger. It went from six characters to now over twenty, and it’s hard for me as the writer to keep up with them all, and for my readers as well.

The problem is, for me as the overall narrator, each character has an important backstory and plot that I need to flesh out. Or do I? I’m struggling with how to write and maintain a large cast of characters. Any tips or advice?