There are how many settings built off of 2e D&D? [MENTION=9849]Echohawk[/MENTION] can likely tell us the answer pretty definitively, but, just for the 2e system, I'm going to guess that the number of published (never mind homebrew) settings numbers in the dozens.
For 2nd Edition, more than a dozen, less than two dozen, depending on how you count the various sub-settings and minor settings:
Major settings: Al-Qadim, Birthright, Dark Sun, Dragonlance, Forgotten Realms, Greyhawk, Mystara, Planescape, Ravenloft, Spelljammer.
Minor or sub-settings: Council of Wyrms, Horde, Jakandor, Kara-Tur, Lankhmar/Nehwon, Malatra, Masque of the Red Death, Maztica, Sanctuary, Savage Coast, Taladas.
Other D&D (sub-)settings not specifically supported in 2nd Edition: Blackmoor, Eberron, Ghostwalk, Hollow World, Hyborian Age (Conan), Kingdoms of Kalamar, Lendore Isles, Mahasarpa, Nentir Vale, Pelinore, Rokugan, Thunder Rift, Warcraft.
That isn't really a definitive answer though. I'm not sure there is one. What counts as a campaign setting? For example, Wikipedia counts Astromundi Cluster a campaign setting in its own right, and there's certainly enough material there to run an entire campaign, but I wouldn't count it as separate from Spelljammer.