The "rule of thumb" that was I told is that you need a gallon per inch of fish. Some of my girls are easily two inches long...so if it was me, in a fifty, I'd do about 20 or 25 girls. This gives them enough space while not overdoing the bioload.
I'm pretty new to sororities, so I totally bow to the more experienced members, BUT, so far all of my introductions (including the initial introduction, and then later additions of new girls), tank rescapes, and even tank moves have all gone really smoothly, with no biting or fussing after the very first introduction day or so. Mine even hang out together in little groups. I think the key to this is having LOTS and LOTS of cover. I found a silk ivy-type vine that's about seven feet long at my LFS, (It's made for terrariums) and has suction cups on it. I bunched it up and suctioned it all across the back of my 20g long, and my guys are all tucked in there every morning when I go to feed them. I think this gives them a sense of security and having their own place which really helps them in general be able to cope with one another and not fight. That ivy is all across the back, and then there's a great deal of silk plants at all levels through the entire tank, to the point that I have to search for girls and count them to make sure that everyone is safe and accounted for. You cannot have too many plants in a sorority, and if you think it's starting to look like a jungle, you're probably there. :)
At least that's my opinion/short experience. :)