I currently have 6, and the aggression is pretty much non-existant now.... if you keep up on water changes and siphoning, I'd say you could keep at least 8 in a 10 gallon...... I actually have 7 girls, but one is so mean I can't keep her with the others...... I don't think my sorority would work with any less than the 6 I have, but depending on the females I'm sure 4 would work out just fine.