There are supposed to be seven dozen crabs in a bushel. That's 84 crabs. Most places don't use the traditional bushel basket; they put them in paper grocery bags.
The number of bushels depends quite a bit on how many of the 20 are crab eaters. One bushel would provide four crabs per person. If you thought they'd eat eight, then I'd get two bushels. You can always pick the uneaten crabs for the meat, which can be refrigerated and eaten later (but not too much later).