I have only one per tank, Dianna, because they will fight to to death with other male bettas. However, they do just fine in a peaceful tank with other fish, as long as you make sure not to place them with other fin-nipping fish.
Bettas are not meant to live in bowls by themselves. If you get a betta, and put him into a good sized tank with water movement, they become the most beautiful fish. They develop muscle tone, and their fins fill out, and they look like they actually care about life and what is going on around them.