It's a recipe for disaster unless the tank is enormous.
In the wild, males have a territory and any fish which ventures into the territory is chased out. This includes females unless they are ready to breed, in which case the male allows her to stay for as long as it takes to spawn, then he chases her away. In a tank, the other fish, including females, can't get away so they are constantly invading the male's space.
The bettas we buy in shops have been selectively bred for many generations to make them even more aggressive than wild bettas - they were originally domesticated for fighting, long before we started keeping them as pets.
It is possible to keep a group of females but they need constant attention. Many members have started well but ended in disaster.
There are other species of betta which can be kept in pairs, or even in groups. These are not as easy to find though. Look at some of the posts in this forum
Wild Types
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