I have 18 in my 55g - They are varied sizes, but 10 of them are small or smaller medium size.
They have a lot of room to climb, dig, eat, swim and play - I am considering a few more, but based on my setup I really think more than 3 or 4 more, keeping the new ones smaller, would be unfair to my guys and gals. I lose a lot of room for a second level because my substrate has to be deep enough in places to accommodate my big guys and one end of my tank is set up to be a salt water "beach" so a second level over that makes me nervous as they would be falling onto gravel or very shallow substrate.
I don't think there can be any set rule for how many crabs you can fit in a certain size tank as there are too many variables - I think the best way to deal with it is to start with fewer crabs and add on over time.
Lady Penelope, D-Dub, Stream, Bakugan, Spongebob, Willie Randolph, Luis, Church, Urlacher, Beltran, Jose, Forte, Wagner, Delgado, MJ, Pippen, Pikachu and Pricilla Queen of the Desert.