Join Date: Oct 2010
Location: Victoria, Australia
I'd think 7-8 females would be a good number depending on the actual dimensions of your tank. I have 6 females in a 54 litre/14 gallon tub, but mine is long and shallow so it provides them with more room than a tank of the same size but different footprint.
Essentially you want 80-90% of your tank covered. You can never make a sorority too crowded or plant it too densely, as bettas are very much out of sight out of mind fish.
By creating a lot of visual and physical obstructions, you are cutting down on a lot of aggression that is the result of females being in constant eyesight of each other. Additionally, heavily planted tanks make it much more difficult for a particularly aggressive or dominant female to corner and attack anyone.
I use terracotta pots, PVC piping and wood to create lots of hiding places for my females. However, I've found these are less important and less utilised, than middle and surface-level cover.
I purchased a mixed VT group of females online that all went into a sorority as soon as they got here. They have been living in there for three weeks now and I never catch them fighting.
That tank was heavily planted from the start, so instead of constant bickering, each female has a specific area that they will patrol and defend. The high plant mass acts as a visual divider, and ensures that no one feels crowded by the others.