The story dictates the number of characters. Nothing is set in stone. You can have a KILLER story with only one character. There is however a thing about having too many characters.
Several writers and I have an impression that too many characters in a short might convolute the story and hinder character development, if any at all. For example, in the Film Discussion boards I am hearing from many different people that a film such as Babel in which stories of several different characters stringed together has much less of an effect than those stories individualized.
Having said that, Babel is an epic of two and a half hours while ten minutes is nothing but a breeze in the wind. With ten minutes, I along with others really cannot see much character development beyond that of a sole protagonist, with the intention that the short film is intended for such a thing. What do you think?