At least 300.. lol
You can keep as many to a tank as your water quality can handle and as long as no fights break out.. skys the limit..
On decifering mono from ocell. I always look at head shape. Mono head shape is more sloped. While ocell. has more of a "forehead" on it. Also true ocell. should have red eyes. And mono should have a deeper cut in the dorsel fin in the middle. Other than that at a young age they're very similar.
But since alot of pbass sold in lfs' are farm bred where they can mix(accidentally or intentionally IDK?) the two species in breeding pools alot of pbass share traits of both fish and can be hard to decifer.