yes, you can pickpocket as many times as you want, but it requires time, i figured it out myself when you pickpocket an NPC for about 5 times and so on repeatition, the next time you use the skill will be sucessful but the inventory of the NPC will not appear, go for some quest hunting, then and after that, return, pickpocket again the NPC, it will show the inventory of the NPC again, i always do this when i want my money back, pick all the NPC's item put it in a container(i always use a barrel), close the barrel then drag the barrel away from the NPC, open the barrel then there you go, you get instant money and items that can be salable to other NPC.