I feed my bettas once a day, in the morning, and skip one day a week (Sunday). I feed 4 betta pellets or 4 bloodworms at the morning feeding. I read once that you should consider a betta's stomach to be about the size of their eye. That's how I figured out the number of pellets. The directions on the container also say 3 or 4 pellets, depending on the size of the fish.
I started to give an evening feeding recently. The owner of the hatchery (where I got the bettas) said they were looking a little skinny. So now, the morning feeding stays the same, and the evening feeding is only a few days a week, Mon-Wed-Fri. I think this gives them a little more food, without overfeeding and worrying about too much food waste. At the evening feeding, they get 2 or 3 freeze-dried daphnia, or 4 Azoo betta pellets. These pellets are much smaller than the Hikari pellets they get at the morning feedings.
Hopefully, this is a balance between adequate feeding, and not creating too much waste.