In a 10 gallon tank it is most common to have between 10-20 hermit crabs depending on the sizes. Be sure to add lots of “toys” for them to climb on such as choya wood, coral and such. The more room you have for them to roam the better, but be sure not to overcrowd them either. Keep an eye on your crabs for signs of agressivness that is uncommon and such. Sometimes they will let you know how they are feeling by their behavior.
Be sure your aquarium has a hood on top to hold in both the crabs and the humidity levels. Some hermit crabs have been known to climb the silicone along the edges in order to climb out. I have two of these climbers in our tank and it is fun to watch, but gladly they have a cover.