no catch limit and you only need a licence if you are crabbing with traps or on national/state parks and wild life refuges. to crab with traps you need a crabbing licence that will allow use of up to 12 traps. crabbing with string and/or nets on public/private land without a licence is no problem.
Actually there is a "catch" limit of 12 dozen per vehicle/boat per day, state refuges/WMA's/National Wildlife Refuges have site specific regulations that you will need to check into.
Warren