my female tank is gunna be a portion of a 10g ( about 7g ), ill post pics later but its rely well planted with java fern and im gunna bury a couple pots in there to.
just kidding... but seriously its dangerous to hav anything under a 10. maybe endparanthesis can pull it off because hes experienced, but if this is your fist female betta tank-definately go with a 10.
I have a female community in a 10 gallon tank, its not planted very much though and they tend to constantly stick by eachother in one spot in the tank, for this reason I wish I got a slightly smaller tank because they hardly swim in more than 5 gallons that is in there. 10 gallons would be perfect for 4 or 5 females with some plants i would say..also depends on how big they are.
here is a picture I took when I first created it and before the sand cleared, you cant even see them expect half of my red female in the right corner lol
i have 2 planted 10s with 6 females each.
if none of your bettas are meanies then you can get away with a tank like bittersweets but most likely you will have some fiesty girls that like to fight.
make sure there are hiding places in all levels of the tank and things to break up "free space"
i have a 15g currently housing 3 female bettas...use to have more until i sepreated some for breeding and left them in their own 1G too long, now they can't get use to have other bettas around learn from my mistake