Bettas With Other Bettas

If you want to have a species only tank, here are some considerations:
No more than one male can be kept in a community tank, unless there is a divider keeping the two males from coming into contact with each other.

Females need to be kept solitary or in groups of four or more in a community tank. A single female will not pick on or be picked on usually by its non-betta tankmates. Female bettas will always fight for dominance when in the same tank together. The important thing is for one female to not be picked on constantly, or for any female to get really hurt while jockeying for her position in the heirarchy.

Three females or fewer will lead to one female being harrassed constantly. Four females spreads the fighting and sparring over more fish, and there is less of a chance for a single betta to be harmed by the encounters. After the initial dance for dominance, females will usually remain peaceful unless disturbed (a move, a new female or fish added, etc). Also, tanks with female bettas need to be heavily planted. This provides cover for girls when they need to rest, or are trying to hide from another female. Java moss is great for this purpose.

Males and females should never be kept together. Doing so will more than likely lead to one of the following: A) spawning, B) fighting and fin damage, and possibly C) death, from damage inflicted by the other betta during the fighting. If you want to spawn your bettas, you need a proper setup, not a community tank. Spawning bettas can be read about here.