I wouldn't go higher than 5, personally.
I have 2 sorority tanks running, currently - a 20 gallon long with 7 girls. When there were 12 girls in there - it was too many! They are all top breathers and the more competing for the top - the more aggressive the environment - and with girls - some are more aggressive than others - so the less aggressive ones will be cowering at the bottom, or hiding ...
please don't overstock the tank when using 'aggressive' fish types.
They really do need room to claim as their own 'space'.
my 12 gallon eclipse has 5 girls and they all get along. It was a try and try again situation to make this tank 'work'. I had 4 girls in there before this weekend, but one of them - was more aggressive than the others, and was forcing 2 of the others into hiding. I switched her into the 'big' tank, and replaced her with two others... and now, everyone gets along.
Just because they're girls, doesn't mean they'll all get along. It's really a very difficult task to find who will be good together. You need to have another tank ready just in case you will need to remove the aggressive ones...