Vampire crabs

I m interested in keeping vampire crabs. Found some info everywhere but still have some questions that don't seem to be answered.
1. I know that they are more land based, so I intend to keep like 2/3 land. As for the water part, how deep can the water be? I read it's 2 to 2.5 inches. Is that good? What sort of filter can I use if it's so shallow?
2. My tank is around 1 foot long. How many crabs can I keep? I was thinking of 2.

Thanks for replying.