Jamie uses 6 packs of beads to decorate 4 belts. Each pack has 8 beads. If Jamie uses the same number of beads to decorate each belt, then Jamie uses a total of 6 packs * 8 beads/pack = 48 beads. Jamie uses these 48 beads to decorate 4 belts, so each belt has 48 beads / 4 belts = 12 beads.
Therefore, the answer is A: 12.