It's good to see everyone else's opinions on this. Far too many people I know just say six because FFVII, XII and XIII had six
In mine, there are 14 main characters and four 'bonus' characters that follow main characters for seperate reasons. Only one is silent, but I've kind of balanced it out by giving him the highest attack/HP of the party so he's really useful in combat situations (He is a pet, hence the silence). The plot is already done and each character has at least one (often two or three) sections dedicated to them. Combat wise, each character is based on a 'class' but often not a stereotypical one (To clarify, the game i'm designing is based off a D&D 3.5 campaign a ran over the last three years), so there aren't really any that can just be kicked to the back and forgotten about. Personally I like a larger party as it allows you to play with your favourites and ignore the characters you REALLY don't like (and we all know we have them). I'll certainly keep in mind the points about spreading the dialogue between all the characters.
Also amerk, that sounds really good about the idea for giving back-up members XP as well. If you could point me to a script for that it would be much appreciated.
Thanks for all the good feedback