Yeah, four seems to be the magic numberfrom the gamesmaster perspective. I personally can\'t handle any more than eight, and even then I don\'t know if I could do it on a regular basis. I have played with only two players before for longer periods of time, and it\'s workable, but they each had two PCs each.
With D&D as a reference, I feel it\'s best if you have at least one each of the \"four cores\": Fighter-type, Arcane spellcaster, Divine spellcaster, and Sneaky-type. So if you\'ve got someone running a multiclassed PC, you can get away with less. (And not get overwhelmed with NPCs...)
Even in the modern-era/sci-fi games it seems to hold true:The Pilot, The Gunslinger, The Tech guy, and the Personality.
That\'s just my two gold/credits...