I'll weigh in here...
I agree, 6 buttons is a pretty fair number to expect per stick. For just old school games (Pac Man, etc.), I can't think of any that would require more than 3, but like the guy said, 6 is a good number for other emulators you may decide to use at other times.
I also see Dave's concern though, in that one can get so wrapped up in trying to have a button for every conceivable manuever that it becomes overkill and the CP looks "busy."
The hard part is deciding if you're going to leave it old school, or expand. If you're expanding later, go with six...better to have unused buttons on the CP than have it all set up and have to try and drill new holes while avoiding existing wires, etc.