A couple of female bettas won't live peacefully. You need at least 4 with more being better.
With two, one will bully the other. With three, two will gang up on one. With four, no one subordinate female will be picked on constantly. But more that four spreads any aggression round further.
All the females should be added to the tank at the same time so they can sort out a pecking order. Any added afterwards will cause problems. And the tank should have lots of decor and tall plants to break the line of site and give the individual fish somewhere to hide.
Some females are worse than a lot of males for being aggressive. These females cannot share a tank. If you find you have a very aggressive female there is no option but to relocate her into her own tank - alone.
For a sorority of 4 females, 20 gallons/75 litres is the smallest I would use. For a bigger group, a bigger tank.