Mine get 1 or 2 frozen bloodworms a day, and snack on mosquitos and other very small insects that I kill or immobilize around the house. Nothing larger than a mosquito, since they can choke or fail to digest large food properly. Live food rules!
Bettas have excellent eyesight- after a few days of "training", mine will jump about 1/4" to 1/2" out of the water to grab a bloodworm I dangle over the water- it's really cool to see!
Agree with Hikari if you do pellets- 3-4/day is good to go as long as they have room to swim. If they act like they're hungry it means they're healthy. Eating food is half of their great lifetime goals- if they do that enthusiastically then you've got a healthy fish!