No standard arcade game uses more than 6 per player. (those that do are mahjong panels, full keyboard, etc)
As was said, more is usually done for console emulation, although I don't like playing games that require me to remember varying functions for that many buttons.
Some of the newer PC games are based around an xbox360 controller and the developers seem intent on using up 8 buttons regardless of how much sense it makes to do so. (do I really need a separate button to pick up objects in a beat 'em up?!)
I'm finding that I just don't bother playing these and migrate toward the ones that use 3 buttons or less, except for fighting games of course.
The most important modern PC games to have on an arcade cab IMO (Mortal Kombat Komplete, SSFIVAE, SFxTekken) only require 6.
It will allow you to map the other two, but they just do the same thing as combinations of the other buttons.
IMO, it is cheating to map those even if you have the extra buttons.
I have 7 buttons on my CP. I do like having the tag button off to the side for MKK instead of in the middle as would be required by a straight 6 layout.
Next time I might reluctantly add the 8th to make sure I'm covered for any future game that might be designed with 8 button fight sticks in mind.