betta's don't NEED friends....i have one alone in a 10gal and he's happy.
i've posted on another thread like this, so i'll just copy what i said...here it is:
white cloud minnows are great! they don't bother betta's, and are too fast for the betta to mess with them....they are really small....so i'd say 5-8ish maybe....someone else could tell ya how many you can have...i'm not good with that stuff.
also a small algae eater would be nice...NOT a pleco though...if you get a pleco and put it in a ten gal, and someone on here finds out....you'll be nagged to high heaven!
i've got a real nice one....a kind that stays pretty dang small. it's called an octo-somethin...i can't pronounce it after the octo....
i've heard alot of people say they have good luck with tetra's and stuff, but that all depends on the attitude of your betta and the tetras....i had surpae(sp?) tetra's with one of my betta's for not too long, and they ate his tail up, he stressed out, and died....all in a VERY short period of time...
some people also have some luck with gourami's....but i think that most people wouldn't recomend it....but like i said with the tetras, it depends on the fish's attitude. some are really nice, and socialable, and some just plain out need a spankin!
i'm not sure about this, but underwater frogs were on the "tank mate" list with betta's....i've never tried it, and i don't hear too much about it....so maybe some other members could fill you in on something like that.
and last of all, ghost shrimp. very tiny, and very cheap....some people have their betta's end up eating the things...but i don't think that it matters very much seeing that they only cost about 30 cent per shrimp....and if the betta doesn't eat the things, then they can go around tidying up the tank a little for ya!
also, cories....i don't know a dang thing about them, but there sure is alot of people that recomend them.
HTH!