I've heard the same thing too, but the minimum for a sorority is 4-5 females. I've also heard that odd number of female bettas work better in a sorority but I doubt there is any scientific evidence backing that statement up.
But then again, the number of female bettas you can keep in your tank is mostly based on your personal opinion and how you want to go about keeping your bettas. If a disease breaks out and most of your females get infected, do you have 10 (1 gallon or more) spare tanks or containers to spare? Would you rather carry a heavy bucket a few times (my bucket is only 3 gallons) or once? And for me, the aggression only lasted less than a week and now they almost never bite each other.