I honestly don't like using caves for sororities. Firstly I've noticed a majority of my bettas rarely use them. When I had a sorority the females spent more time in the upper levels of the tank than they did the bottom.
Also, with caves that have only one entrance, there's nothing to stop a female from following another one in there and starting a fight. This is why I prefer the use of plants (whether live or fake) as cover, as they provide an actual physical obstruction.
Opinions on how many females to have in a sorority vary. I am someone who overstocks sororities as I feel it allows for a more even dispersal of aggression. I would say you could have up to 20 females in a 20 gallon tank. However, if this is your first sorority I think 10 would be a good number to aim for.
Here is a thread I posted on sororities. It provides some additional information.
I just want to say that it is very important you quarantine all your females separately for at least a couple of weeks. Disease is a big killer of sororities and even visibly 'healthy' females can be sick.
Also, is your tank cycled or are you going to be putting your females into an uncycled tank? Maintaining water quality is crucial in sororities. For these fish, living in a group can be stressful enough, without the added stress of deteriorating water quality and the effects of ammonia/nitrite poisoning.