I have somewhere between 6-10. I'm not sure, because there's always some buried.
Whatever you keep them in, you should be able to maintain 70-80% humidity. This will mean a 10-20 gallon aquarium (long is better)
You can use either play sand (moistened with salt water to sand castle consistency) or Eco-Earth (moistened with 80 oz. of salt water to the brick)
Keep your temperatures in the 75 to 80 degree range. Under tank heaters or reptile daylight/moon glow bulbs or a combination of both will help to achieve this.
Put things in the tank for them to climb. They're known as "tree crabs" in the wild.
There are plenty of commercially prepared foods for them, but you can also feed them things from your kitchen.
There are several good hermit crab sites and forums on the web. Check a few of them out.