Some people go OTT with buttons on their cabs trying to cover too many bases.
For maximum flexibility you'll want 6 buttons per player and a P1&P2 start button (on a 2 player panel).
If you're only ever going to use your cab yourself then admin buttons can be easily done by using the 'shift' key method supported by most encoders, where holding down a certain button (like P1 Start) will allow you to use other buttons for Pause, Escape etc.
If your machine is going to be used by other people its usually helpful to include a dedicated Pause and Exit button as a minimum.
My current machine has 6 per player, P1&P2 start, P1&P2 coin and admin buttons for Pause, Exit, TAB, and Return.