A bajillion. Actually just over a thousand, though I can’t say I’ve tested very many as I’ve only focused on the few that retropie doesnt have or that aren’t obscure enough that nobody would want. I’m guessing some just flat out wont work, others probably will have terrible performance, and there may be a few that perform well. If you have the time for testing I’d be interested in your results.
Just as an fyi MESS is more of a quantity over quality so they basically just puke out as many barely functioning systems as they can without too much regard to optimisations and whatnot. Standalone emulators and retroarch focus more on performance and optimisations hence why they usually run better than their MESS counterparts.
When I’m feeling really ambitious I may make a module for a few systems but I need to figure out if I want to mash it in with advmame or if I want it as a separate module with multiple system configurations. I haven’t really decided yet.