Hi,
I have 7 buttons for each player, P1 & P2 Start, P1 & P2 coin, plus mouse buttons one and two on either side of the trackball.
Overall this is ample so long as they are placed conveniently to use with your controls (joys/T.ball/spinner).
For all the other functions I just use the P1 Start button as shift. It keeps your panel relatively uncluttered and for the amount of times you need a shifted function (pause, escape etc) I didn't see the need to fit a dedicated control. Running win XP with MameWah.
My start button is leaf switch arcade button mounted vertically just below the left hand side of the bezel along with the volume control where one of the coin slots used to be. I just ran the wiring from the pins on the motherboard to the leaf switch instead of to the PC start button.