The first step in an old fashioned crab boil is to gather your crabs. You can buy bushels of live crabs in many Eastern markets, and on the West Coast, you can sometimes find live Dungeness crabs. These here are Pacific red crabs, rock crabs and Dungeness, collected off a jetty in Northern California.
Figure on at least a half-dozen blue crabs per person, eight red crabs or calico crabs, 10 rock crabs (most of the meat is in the claws), or 2-3 Dungeness crabs. These are general guidelines, so adjust to your eaters' preference.