How many bettas can be kept together in the same aquarium? That answer depends on the sex of the betta. Only one male can be kept in an aquarium, as males will fight with one another (hence their common name, Siamese fighting fish). In the wild, one would retreat. But that isn't possible in an aquarium and so they continue to fight, often to the death of one or both.
Females are a bit more tolerant of each other. As many can be kept together as there is adequate room for in the aquarium. However, if it becomes too crowded they may start showing territorial behavior.
You should not mix males and females in the same tank, other than temporarily for breeding purposes.
Betta lovers will often use a "betta condo" to allow them to keep multiple males in a single aquarium. The condo is simply a small container that is vented to allow water circulation through it. It hangs inside the aquarium, effectively keeping any fish inside separated from the rest of the tank.
A certain degree of controversy surrounds the use of betta condos. Single condos kept in separate locations within the tank are acceptable. However, when several are kept side by side it can induce males to flare at one another. Some owners feel that the stress caused by this practice can negatively affect the health of the fish.
Does Tank Size Matter?
Some betta owners have found a degree of success with having multiple fish in a large tank. An example is Natalie, who had two males and a female in a 39-gallon tank. She noted that they chased each other around a little for the first few days, but then settled down and lived together peacefully.
The species is native to the Mekong basin in Southeast Asia, living in rice paddies and canals. In the wild, more than one male would live in a rice paddy. However, rice paddies are quite large, often encompassing miles of space. This allows each male to have his own territory. In small tanks, there simply isn't enough room for the establishment of territories, so it is not advisable to keep more than one male.
Somewhere around 20 gallons is the point at which size becomes a significant issue. Sizes smaller than that are usually a problem. Sizes larger than that provide enough territorial space to allow multiple males. However, few people keep bettas in larger tanks. Traditionally bettas are kept in very small tanks, hence the common statement of "one male per tank." A more accurate way of putting it would be one male per territory.
Another factor is that the domesticated bettas are the product of selective breeding for heightened aggression. In Thailand, they were collected specifically to fight in competitions. They are more likely to keep fighting, whereas the wild breed would spar briefly and then retreat. This makes it more necessary to give each fish a territory of its own.