Doggyhog pretty much covered it. I have 6 girls in my sorority.
It can be difficult, and make sure you have aquarium salt to treat fin rips! There WILL be rips, which may be severe enough to need treatment! You also need to make sure you will be home pretty much ALL day for a few days, so you can watch them, and separate any overly-aggressive females.
Minimum, like doggyhog said is about 4. But odd numbers are better. I don't know why, but they are. Even numbers can be done though, just not as high success. Maximum is probably 6 or 7.
I wouldn't put the females you have now in the future sorority, especially considering they were so violent they killed another betta.
Research is your best friend ;)
More hidey-holes and plants and such the better!