Put some water and a steamer insert in the bottom of a very large pot. Add the crabs, sprinkling the spice mix and the herb sprigs between the crabs as you layer them in the pot. Put the lid on, bring the water to a boil, and steam until the crabs are cooked, about 20 minutes. The crabs should be a bright red. Remove the crabs from the pot, serve hot, and pass around crab crackers or hammers.