I have so many characters in my head that I don't know how to use them all.
Have you ever tried writing some poetry? Poems are like living characters.
Yes, I could write myself some poetry about my characters.
It's good to be able to come up with many good characters, even if you can't use them all, as it shows that you are a very creative person. If you are under a constraint and can't use all the characters that you have come up with, you should try to select the best options from that group. It's always possible to save those other characters and ideas for another situation.
I'm writing a novel with all these characters.
That is good that you found a way to incorporate all your characters. A novel is a good place for that since a story can have multiples.