In terms of "social dynamics", there does seem to be something about 4-6 people around the table. This applies to various situations. Its seems to be enough to keep the conversation going, but you don't split into side talk as easily (though you push it at 6).
(for some situations, like running a large organization, you can go up to 9, though probably not more, and of course 1 on 1 is a special case...3 though, 3 seems like a weak number...)
For gaming, I can run with 2 players, but you feel the drag in terms of energy, 3-5 is better. For characters, I really like henchmen, though the games action economy has to allow space for it. (As Merric has touched on, 4E is interesting here, since the math is key, but also clear, and you can get that right. With that, you can make effective supporting characters with just a few powers, which just confirms the massive diminishing returns that 4E powers have even at relatively low levels).
And I do mean henchmen (or retainers or cohorts). It has to be a clear subordinate, and the player should have only one "main" character.