too many characters?

I have an idea for the 25th anniversary story contest. The problem for me is that it will mean writing twenty or more characters. I don't like writing "spear carriers". I try to have some feeling of every person in the story even if it isn't obvious (Did anyone catch the leather biker's "craft" magazine in The Worst Day?). I'm afraid of having four people and a bunch of cutouts. That's the kind of story I have trouble finishing. How do any of you deal with making secondary characters real?