I'd guess that 15 would be a nice number. That gives them about 3 2/3 of a gallon each. Of course, make sure it's heavily planted and has lots of caves and hidey-holes. In terms of bioload, you could put in 27, but I don't think that would give them enough space.
But there's so much more that you can do with a 55 gallon! You could toss in 7 bettas and still have lots of room for at least 30 more inches of fish (in a nice, easy to care for, understocked tank). How are you even going to find 15+ female bettas?