me personally i did 8, for two reasons, first is the vectrex, my fave system. second i think 8 symmetrical buttons helps with console emu's. here is my layout:
1 2 3 4
5 6 7 8
take snes for instance. i map my buttons 2,3,6,7 to the snes face buttons. then i map the shoulder buttons to 1 and 4
playstation, same thing, face buttons to 2,3,6,7, shoulder buttons to 1,4 and 5,8