Depending on the size of the pellet, you can feed about 4 - 6 pellets per day, broken down into two feedings. If your betta looks a little bloated, you can stop feeding him for a day or two, then start back on a normal feeding schedule.
You can also supplement his diet with different brands of pellets, frozen worms or shrimp (look for a freezer in the pet aisle). There are a lot of different opinions on feeding. I usually use brine shrimp as a treat, meaning that they may get it once a week or so.