i think it really depends on the classes, for example, in DAO, as you have just 3 classes you can go with 4 guys in the party and have 2 warriors(tank/dps) , 1 rogue(thief), and a mage that is both dps and healer, so you dont need more people in the party, but with more classes, if they're truly different, you' need more people because each class will have a very specific role.
if there's any mistake that doesn't allow you to understand what i mean, forgive me, i'm spanish