I voted 6, as that's what would be in "Dave's 4e D&D" (cleric, fighter, noble, ranger, rogue, wizard), but I've never been too consistent on this. I thought the 3-class setup in True20 was compelling in theory, though not so much in actually reviewing the rules. SWSE got the setup right for Star Wars, I think (though I have a house rule proposal that nukes the Jedi class). But in 3.x D&D, I've always liked toying with WotC's new base classes.
So I guess I'd really say the 7 +/- 2 theory works for what should be available for any given game, but you might vary the actual classes allowed even from one D&D campaign to another.