Hi
If its just female betta you want to keep, then a good number to start with is 5. Then any aggression will get spread out amongst them, hopefully.
A 10 - 15 gallon tank, heavily planted to provide shelter should be adequate.
In my experience, I found that by slightly overstocking, the aggression lessens. I wouldn't necessarily advise people to do this, but this is what i found. Obviously, filteration has to be upped.
All females should be released at the same time. If any are added at a later date, it is good practice to remove the ones already in the tank, rearrange the decor and release the new ones then the original ones. If any of the original ones was more aggressive, this one should be released last.