remember, not all females will get along. they're still fighting fish. my CT had to be removed from my tank with three other girls because they were always picking on her and she'd just go hide. the three are now getting on quite nicely but not all of them will get along.
but remember, an all female tank has to be heavily planted/decorated so if they feel like they need to hide, they can.
i'm thinking around 20-25 girls? i know in a 10g you can keep up to 8.