8 buttons makes sense for Steam and Xbox360 games, assuming that the controls show up as an xbox360 controller. Some games like Mortal Kombat X make you map every button on the x360 controller before allowing you to exit the remap screen. Castle Crashers uses every damn button. If not playing Steam or Xbox360, there is no reason to have more than 6.
Steam or Xbox360 games that use analog thumbsticks usually use both, so for the most part they aren't going to be playable anyway.
You are pretty much limited to fighting games and platformers that don't use analog. Anything else is a PITA to try and play with the arcade controls. Even with the platformers, it gets annoying if you have to remember what more than 3 buttons do. Quite a few of the newer platformers use the right analog thumbstick for aiming of some sorts, which makes them unplayable with arcade controls.
If you must have pinball buttons, do 2 on each side.
Some games require it and they can be used for "nudge" on games that don't.
When I put together my first system, I put dozens of emulators on it so it could play "everything".
The majority of them went unused after testing them out.
Most games for anything N64 or newer were unplayable because the controls became analog and complicated moving forward.
When I got around to building an actual cab, I decided to go arcade only/no pinball.
IMO, time is better spent installing non-MAME arcade stuff than Atari 5200.