Psion
Adventurer
Delta said:I'm big on the "7+/-2" rule. That's a quick psychological rule-of-thumb for how many options people can retain in their short-term memory to select from. My own interpretation is that 9 is beyond the capacity of lots of people; 7 is manageable by most people but takes some work; 5 is doable by everyone and simple and fun.
http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two
Heh. Cute.
Also makes me think of a management rule of thumb. The idea is that a manager can generally handle 7 subordinates within a hierarchy; the ideal number of layers of management equals the log-base-seven of the number of employees.
As to my answer, I do find 5-9 to be about accurate; it really depends on the setting, genre, and tasks implicit to the campaign setup. D&D usually needs the classic 4, though usually one GMs or another's style might press a 5th class. A game like traveller usually needs a pilot, navigator, gunner, security specialist, engineer, and medic, with a "faceman" (carousing/liaison levels somewhere in the mix).
Those saying zero/ "I love skill based systems": I find that regardless, a competently and effectively organized group will have a similar number of core roles (you'll note that I used Traveller above despite lacking classes in most implementations.)
I really think that in 3.5, the number of base classes is entirely out of hand.