The folks at lowcountrycatch.com Seafood say, "For example, if you are having 40 people to your roast and are also serving chili and hot dogs, I would order six or seven bushels of oysters. If you are having 40 people to your roast and are serving nothing besides oysters, I would order 10 bushels."
A bushel is about 50 pounds in the shell, unshucked. This is from 100 to 200 oysters, depending on size (usually closer to 100).