I dont think theres a limitation, but Avid will create new folders when neccessary. BUT Id suggest asking on the avid.com community as someone there will know exactly what the limits are for you rather than my guess.
What Michael says. No subfolders for OMFI MediaFiles. I've had folders with 15,000 files in them, but you don't want to have to rebuild the media database for such a folder. Also, the OS will become very slow when opening folders like that.