How many buttons should I have on my arcade machine?

I’m making an arcade machine. It’s going to run off emulator technology, which I won’t delve into too much.. but sufficive to say I’ll be able to put multiple game systems on the arcade machine. Along with a litany of classic arcade games I plan on including a number of home console systems (and many games for each). Right now I’m planning on including NES, SNES, Sega Genesis, N64, Game Cube, and Dreamcast. I may end up including playstation 1 and/or 2 as well.

The machine will have joysticks and buttons for four players, to take advantage of the 4 player arcade games and console games. But here is the question, how many buttons do I include for each player. The N64 has the most buttons on it’s controller of the systems I have listed (9, not counting joystick). But not all buttons are functional for most games, or they do some obscure thing that you’ll never need.

It’s a bit of a balance issue, if I have too many buttons it’ll be crowded and difficult to manage. There’s also an issue of how fun will this actually be on an arcade cabinet as opposed to the way it was originally meant to be played?

What do you guys think? The minimum number of buttons for each person is 6 action buttons and two auxiliary buttons (add coin/start).