@Simone This answers that and tank size. Peace.
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). Females are a bit more tolerant of each other. As many can be kept together as there is adequate room for in the aquarium.
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 it 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, including myself, feel that the stress caused by this practice can negatively affect the health of the fish.
http://freshaquarium.about.com/cs/qa/f/faq0011.htm