I have found that it's a good idea to have several empty shells per hermit.
Depending on the hermit species, this can especially true if there are any snails in the tank.
Many hermits will spend the day eating...and looking for a new "home". Some hermit species will look for empty shells, while some will actively remove snails from their shells looking for their next "new digs". The Blue-legs that I've kept were notorious for killing snails for their shells.