In D&D 3.5e and the official expanded books, it seems Greyhawk has about 10,000 different gods; some just mentioned in a table somewhere.
There's nothing that I can find in the rulebooks, but I read here in a wiki that some "gods" cannot grant spells, such as Rank 0 and Overdeities. However, I've seen nothing in any rule-books that define which "Rank" any god is.
So I'm unsure if this was just made up by a wiki author, or maybe I'm missing something.
In particular, there are monster deities, such as Maglubiyet, which from what I can tell was only briefly mentioned in errata for MM1, that I can't really tell where they sit in the default Pantheon.