Most fighter games require 5 or 6 buttons per player, but otherwise very few games require more than 3. Defender is 5.
Some people go 7 buttons, with 4 on the first row, and on Neo Geo games they change the layout in mame so the first row is 1-4 instead of 1-3+7. It would look like this:
1 2 3 7
4 5 6
Another layout option is to go like this:
1 2 3
4 5 6
7
That way you get 2 rows of 3 for fighters, or a 4 way "d-pad" type option. Plus button 7 can be your shift for admin controls.
For admin buttons, the start buttons are key. You can get coin buttons with shift commands if you really want to keep it minimal, but I personally don't like too many shift functions because guests never know what to push. Coin 2 is required on some games to play the player 2. If you have only a coin 1, then making something like player 2 start a shift button will allow you to make a coin 2. But you will want an exit button too, and if you only have one coin button and 2 start buttons, exit would probably have to be the p2 + p1 start to trigger, but that can cause problems on 2 player games if you both hit start at the same time. You can use player buttons with the shift as well, but again, it is too easy during frantic gameplay to end up hitting your shift button and a player button and inadvertently triggering an admin function.
I would recommend dedicated coin buttons, but you can hide them under or on the sides or something if you don't want them on the CP. I would also recommend 1 or 2 admin buttons (exit and pause are good ones to have). I would also suggest two buttons near the trackball as mouse buttons too, but if you lay things out well, you can just use the P1 or P2 buttons to play with the trackball, depending on if you are right or left handed. Mouse buttons aren't critical, but wow they come in handy when something needs adjustment or something fails to go well (or if you spend any time working on your FE at the cabinet)
Look at slagcoin.com for button spacing and layouts, print out one you like and set it on a surface at the height you plan to put your CP, stand in front of it and lay your hand on it.. if the buttons line up with your hand well, then go with it.. if not, you might want to make adjustments to prevent fatigue.