As you can see it's not really a case of "... get away with 4..." as having a MAME cab is not a definitive thing - it all depends on what you want to play. Here is a quick summary of what some have said:
- 2 buttons each player - for simple cabs with older games
- 3 " " " - was the JAMMA standard, and will cover most games up to the 'fighters era'
- 4 " " " - was the Neo-Geo standard, so is great if the most you wanna play is Neo-Geo and some basic consoles too.
- 6 " " " - will cover most bases including all fighters and if you want more consoles (SNES etc.)
- 7 " " " - great if you are a Neo-Geo officianado as you can have the Neo-Geo 'feel', but overkill for your average gamer or those not used to genuine Neo cabs.
- Admin buttons - at least a start button for each player and one coin input, even if just from the coin door. If running other emulators you may want to consider adding a credit button for each player, and if you want convenience and have room, add a couple more buttons that may come in handy for FE/emu tasks like escape and pause, and they could even double up as rarely-used buttons on more advanced console emulators.