That really depends on the size of the food and how often you want to feed your Betta. It also depends on the temperature in your tank. The cooler the setting the slower the metabolism which will make it difficult for him to digest large amounts of food. The best foods for Bettas is the Omega 1 Betta Pellets or the New Life Spectrum Betta Pellets. These pellets have less fillers that are often to blame for Betta's digestive problems.
New Life Spectrum also has garlic in their pellets, which helps keep the Betta's immune-system strong. There are small sized Pellets and large sized ones. 4 big pellets or 6-8 small ones a day, half the portion if fed twice a day sounds about right.
No more than once a week do I give my Bettas frozen bloodworms. My Bettas like the pellets soaked in tank water before I feed them, or I grind the pellets into smaller pieces. Should your Betta ever get constipated please do not give him peas, as the high fiber content is to hard on his system. Better to fast him for 3 days and then feed him frozen (and thawed) daphnia. They work as a laxative and should get your Betta back to normal.
Hope this helped