FWIW, the mame driver is still in progress, and the game is just using a generic-for-that-system input list. Of the 30 games in the driver, there are only four different input lists, including the generic one, and only four games don't use the generic list. IOW, 26 games in the driver use the generic 6 buttons input, and some might have used less buttons. Add that older World Statdium games that were on older hardware systems only used 3 buttons.
IOW, the driver might say six, and the hardware was able to do six, but the game might have used three. So the answer is "Maybe you only need three buttons".
