Just one betta fish. No matter how big the tank, it should not house more than one male betta fish. This is because bettas are very territorial and aggressive, and will fight each other on sight. One or both fish may be injured, or they may even fight to the death.
While not as ornery as their male counterparts, even female bettas can be aggressive. For this reason betta fish are best kept in single-specimen tanks, or with carefully considered tankmates.
However, there are a couple of exceptions to this rule. Some betta keepers partition their tanks, meaning they use a separator so they can keep two male bettas in the same aquarium. It’s a clever idea – you can have two fish, but only have one aquarium to care for.
Using this method you could keep two bettas in a 12-gallon tank, but be careful to choose a divider that doesn’t allow them to see each other. Otherwise, they will constantly try to get at each other, causing a great deal of stress for both fish.
The other exception applies to female bettas. Some fish keepers have several females in the same tank, and these are commonly called sorority tanks. Remember that females can still be aggressive. You’ll want to have a least four of them so a single weaker fish doesn’t take the brunt of the abuse.
Four to six is a good number of female bettas in a 12-gallon tank, but make sure you do some research before getting started. Betta sororities can be tough to manage for beginners.