Join Date: Oct 2010
Location: Victoria, Australia
Like MattsBettas said, you can go up to 20 in a 20 gallon provided you have adequate cover and your filter is able to handle the bioload of those females.
Many people are conservative with how they stock their sororities, but the more females you have the more the aggression is dispersed rather than focussed on individual fish. It's the same theory I find many cichlid keepers work on.
However, in a sorority the importance of quarantine cannot be stressed enough. Even if all your females come from the same source I advise a minimum quarantine period of two weeks. Sororities (even the best of them) are a stressful environment and if you put a female that hasn't been quarantined into your sorority you are taking a huge risk. Many times, if you look at cases where sororities have fallen apart due to disease it is because of improper quarantine procedure.