If you don't plan on playing playstation, x-box, or other consoles that require alot of buttons, then you should need no more than 4 buttons for p3 and p4 (Really, you shouldn't need more than 3).
If you are going to play console games in your cabinet, then you would need all of the buttons required for the console.
This gets crowded though, so you should consider just using controllers for p3 and p4 if consoles are in the picture.