sir_impeccable
21 MCs is a lot... you can keep the plot with several of the 21 being side characters. A main character requires a lot of time and detail to create an attachment to them.
A real life book that comes to mind is āThe Hunger Games.ā A fictional death match between 24 kids is the plot. Of these 24 only about 6 are even named. Of those, 3 or so are main characters.
For your book two have 21 main characters, it would have to be very long and everyone would have to be mentioned often. Sort of like the avengers. Each super hero is a main character, but they each have their own entire movie series. When then group up for avengers movies, each one of them has 1-2 movies that happened since the last avengers movie. You care about each because each is really well-developed.
Now, look at the DC equivalent with the justice league movie. They released one bad movie for some of the main characters, and no one liked it. No one cared for the group because they hadnāt built up the characters.
Just a thought, hope it helps.