Max filesize for a folder is 30mb. Anything over that and hackchi won't even let you sync.
I once crammed a bunch of misc roms into the retroarch folder, when I was a newbie, but now I do it the 'proper way' and run shortcuts with command lines (saves space that way also). Killer instinct won't fit, lol, I know that much.
Oh, maybe you are asking how many you could fit number wise in a folder (let's say if they were all really tiny and not over 30 mb total). Good question. Probably no limit. Might be worth experimenting with.
Time to cram 1000 tiny roms into a folder. rofl.