No problem bro. I'm not sure of the limit, I put 3 arcade games on 1 folder last night (Sunset Riders, Outrun, TMNT2) which was about 44MB and I didn't have any issues.
I would still stick to about 30-40MB per folder but with these there's no saving so you might be able to fit more.
I'd be curious to know the limits as well. I was just watching a MAME2003 video (the one I linked above) and I can't wait to try some more arcade stuff!
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I believe you need to "bind" you buttons so that A on your controller is actually A in Retroarch. You need to so this with all your buttons.
Go into the RA menu, then access the bindings in there to change them.
I have 19 MAME roms on my NES. 10 in one folder, 9 in another. No issues.