When Jack and Kate are trapped in the net, Kate takes a shot at the rope to cut them loose. Then, while Jack is aiming, he says that if he misses, they will have 13 bullets left to shoot each other with. Therefore, there were 15 bullets (13 left in clip + 1 used by Kate + 1 used by Jack) in the gun when they left. Also, in the previous episode, titled Dave, Sayid takes a shot at Henry. Therefore, there were 16 bullets in the clip at that time. Both 15 and 16 are one of The Numbers.