That all depends on how much space/quality you want each betta to have, how many dividers you are willing to buy and clean around, and what gender you would like.
As a general rule, bettas like long, shallow area of water. So while you could divide your tank into 10 one gallon strips, it is doubtful that the fish will be content, or have adequate airspace. Usually, 3 or 4 dividers is the maximum amount for a 10 gallon if you want to maintain a decent habitat for each male betta.
If you want to spoil two boys, split the tank in half, and give them each 5 gallons to enjoy! One gallon may be the minimum space for the species, but 2.5-5 is generally considered best, and the fish certainly do appreciate it! Or, there is always the option of getting only one male betta, and spoling the heck out of it. I did this for my male betta Dickey Barret, and he greatly enjoyed the space - used every inch of it! However, I can understand if you don't really want only one fish in a tank that size.
Now, if you'd like several bettas and no dividers, you can go the route of females. I've heard that 4-5 females in a heavy planted 10 gallon tank is your best bet so far as them getting along goes. Be sure to either buy them all at once and introduce them all at once so you don't have to rearrage the tank and upset the heirarchy with each addition. If you are buying from a store, look for ones allready in a community tank together; females who are not used to community life can be much to agressive to live peacefully together. And remember the point of heavily planted; betta girls can be downright nasty, so there needs to be a variety of hiding spaces for them in the tank. And, you'll want to keep in mind that if one does get severely bullied, you'll need a back-up habitat for her.
I am going to suggest that if you get bettas for your tank, get them from a breeder. There are many wonderful breeders on this forum alone, and many breed varieties with smaller finnage. I don't know what current you have in your 10 gallon, but some males can not handle a strong current, and the bigger the tank, the quicker it usually is. Bettas from reputable breeders tend to live longer, healthier lives, and come in a variety of beautiful colors, shapes, sizes, and types.
Good luck with getting your bettas, and however you end up arraging it, I'm sure you'll enjoy the company of this fascinating species!