No need to get a bigger pot, especially if your pot has a strainer basket in it. Just make your first batch, with all the trimmings, and as your breaking into the first crabs, bring your water to a boil again and add more crabs. They will cook while your working on the first batch. I don't think 12 crabs will do it for 4 people. Not a whole lot of meat on those suckers sometimes. I usually plan on 6-10 crabs per person, but then again, when I do things like this it's not often so we tend to overindulge. I tend to agree about skipping the beer for crabs. When doing shrimp or even crawfish I find it adds to the flavor, but for crabs I find it doesn't add enough to make it worthwhile, so as shroom said, keep the beer for yourself.
Up here in Wisconsin, where fish boils rule (personally I think they are terrible) they have these boilers that are about the size of turkey fryers, or maybe a bit bigger, but they contain a large strainer basket, almost the size of the pot itself. Makes it real easy to pull everything out, and then doing crab, shrimp, or crawfish boils it's great because you can reuse the already hot water, and not have to wait for a new batch of water to come to a boil.