It depends on the writing, but I've not played many games where there's been a mass of characters that all feel different. The Suikoden series, with it's 108 Stars of Destiny, does it's best. As does Radiata Story with it's list of 144 friends. However, I've found that, 90% of the time, once the characters join the party, they loose all the charm/personality they had before they became party members.
So, in the end, I would recommend a relatively low number. The most I've tried to write for is six, and even then, I firmly believe that only one of those characters was written particularly well.