I recommend the aqadvisor for your stocking options. It is an easy to use Tropical Fish aquarium stocker that will give you a very good idea for how many fish vs the filter you have.
I have 4 bettas and one ghost shrimp in my 10 gallon. It has been fluctuating between the 4 and 6 bettas, but the last two have been too aggressive for the tank. I have it moderately planted (7 in total), and several caves and hide spots (also 7 in total).
I originally had all the bettas together. This nearly killed them all because the tank was not cycled.
I seperated them all into seperate bowls, and two weeks later, reintroduced them all back into the aquarium slowly.
I decorated the aquarium first of all.
Then I added the weakest fish, and let her get to exploring. I then added the next two that were about equal in size and color vibrancy. These two established their pecking order, and about a day later, I added another and a ghost shrimp.
I dropped the two largest bettas in on the third day.
The final two dominated the aquarium, and one was far too territorial for the aquarium's balance. I've since removed her, and the aquarium has been very harmonious. I also had to remove an injured betta that got caught in a bad decoration :(. SO I'm currently down to four in the tank w/ one ghost shrimp.
I might reintroduce the other 2 bettas again, but as for now, the one is far too maimed, and the other is a little too dominant for the tank.
I would recommend getting at most 4 to begin, and make sure your tank is cycled.
Also make sure you have a couple of backup options, at the very least 3 holding bowls for the bettas so you can find configurations in case there is one (or more) very dominant females.
keep your eyes on the tank at least for the first hour, and if they get too rowdy at the beginning, turn off the light.
I found the easiest way for them to get along is to keep the tank (And room they are in) very dark. By the next morning, they've gotten a bit more used to eachother.
Best of luck, and please post your results as you come across them!