Except for fighting games and a few shmups, console games don't play well on a cab once you get to the generations where the controllers had analog thumbsticks.
6 will do. 8 if you want to future proof it for Steam games and newer consoles.
You can get by with 6 for all the current fighting games.
The extra 2 only post combos of the other buttons, but who know what will happen down the road.
Personally if any game other than a fighting game has more than 3 buttons, I don't enjoy playing it on the cab.
I have 7 on my cab and like having the extra button as a dedicated "tag" button for games where you can play tag team.
That way I don't have to remember the combo for tagging.

I find myself playing a lot of Steam games, so I'll probably add the 8th button on the next incarnation.
I'd make the last two black or grey so they feel like they are extra and not part of the core 6.
I also miss the anchored feeling of concave buttons, but think they'd look weird with more than 6.