It's suggested to feed him twice a day, maybe 3 pellets each time. 12 pellets is definitely too much. A betta's stomach is about the size of his eye. When they overeat, they get something called "SBD", which causes them to lose control of their buoyancy. While generally not dangerous by itself, a bad enough case CAN cause death if they can't reach the surface.
Sorry to start off with such a scary thought xD Welcome to the forum! Pictures are loved, so be sure to post some in the photo gallery (: