I have 4 hermit crabs and i have 6 shells in there cage is that enough?

that should be enough, because hermit crabs do not all molt at the same time. A little tip is to not put shells that are exactly the same size and shape in the cage it leaves more options. Also when a hermit is molting it is a great idea to separate it from the fellow hermit crabs, because it is vulnerable during this process so by separating it you are protecting it.