Well I'm planning on getting a sorority for Christmas as well, except I need pretty much everything xD From what I've read and heard on this site, 6 is a good number for a 10 gallon, although no less then 4 and no more than 8. Also have extra tanks nearby when you put them in, in case 1 female is TOO aggressive and needs to be separated. There will be nipping and chasing when you put them all in together, but they will establish a pecking order, and one female will be Alpha.
You also need to have enough hiding spots for each betta, so I'd suggest a few caves or pieces of driftwood, along with tons of plants (live or fake). I'm going with a Desert Theme for my Sorority so I'm getting Sand, an Egyptian Vase they can go in and out of, and at least 8 live plants of all different species. Good luck!