I went to 4 colleges, applied to a number of others (not all US based), taught at a few and am giving advice to friends kids now going to college. With minor variations (3-digit v 4-digit course numbers) I've never, ever seen a college course with a single digit course number. Or 2 digit. But then, I've never met an English speaker so culturally tone deaf that they would question the idiom "Foo 101" as being anything other than "intro to Foo".