Honestly, when writing stories with a large cast of characters, I find it best to write out a few chapter to see where things go. Often times the characters will, in a way, cast themselves. What I mean by this is that some will turn out to be main characters important to the plot, while others become supporting characters, there to help the main cast. It's always a good idea to find out who the core characters of a story are before delving into it as a whole.