Flakes are generally not a good way to feed bettas. They generally offer poor nutrition and can cause bloating and constipation.
I suggest switching to a high quality pellet food, I use OmegaOne betta buffet pellets, but Atison's Betta pellets, Ken's Betta Crumbles, and New Life Spectrum are also good brands. Always soak the pellets or flakes in a little tank water before giving them to your betta. Fish bodies were not made to eat bready air-filled foods, and not soaking the pellets can result in bloating--soaking the food is an easy preventative measure.
3-4 pellets once a day should be fine. It is a good idea to vary your fish's diet with other brands of prepared food or with thawed frozen foods a few times a week because no single pellet can provide your fish with complete nutrition.