2, shot and bomb.
You forgot "magic" - so three!

Actually, that answer is dependent on if you want to support console emulation past the SNES. So here are some short answers (all assume a separate Start/Coin button):
Six covers most, if not all MAME games (not including special controllers, majong, etc.) and most classic systems up to SNES (assuming you use COIN as the Select button).
Seven buttons are usually only for those who want to combine the classic Street Fighter layout with the Neo-Geo layout of four buttons across. Like so:
x x x
x x x
x
Sometimes that seventh button is wired to the same input as the first button, so it's still only six "dedicated" buttons. You can do it that way or make it a separate seventh button.
Once you hit eight buttons, I assume you're looking to play PS games or later.