It's really personal preference. I used to have ten or eleven females living without issue in a heavily planted 10 gallon tank.
I have always found the more females I had, the less aggression I ran into. However, the long-term success of a sorority is always going to be dictated by your set-up and the females you select.
If you are going to 'overstock' a sorority, make certain your tank is cycled and the filter is able to handle the bioload. You do not want to be having ammonia or nitrite spikes in a sorority as that is when you start to run into issues.