How many shells do you need for a Hermit Crab?

Hermit crabs change shells once and a while so put about maybe 3 or 2 per hermit crab.I have my own hermit crab and i thought this question.But my hermit crabs havent been changing shells a lot.And make sure you use the right size because hermit crabs dont change shells if that shell is to small or to big or have holes in it.Have Fun With Your Hermit crab