I've come to the conclusion that the perceived need to draw up a cosmology and reveal it to the players drains a lot of interest out of the religious aspects of a D&D world. Rather than saying "my campaign has 5 gods and they are X, Y, and Z, and have 1, 2, and 3 domains," I prefer to say, "in this area, there are the following player allowed faiths. They generally believe this, that, and the other, and you can have these domains. There are also a number of belief systems that you know of, but which are not allowed to players. They are said to believe A, B, and C, and demonstrate powers 4, 5, and 6. There may be other people in other parts of the world with other powers and belief systems." IMO, Paradigm Concepts has done a pretty good job of this in their Arcanis setting as countless "is deity X really deity Y who is also masquerading as deity Z and is in league with mysterious power E"?" threads on the Arcanis yahoo groups attest.
For the question, however, I the relevant answer is the minimum number of gods in a pantheon rather than the minimum number of gods period. And that number is probably two. Less than that and you're monotheist/henotheist. Any more than that and you're past the minimum. I think the earth mother and the sky father, the two gods of Zoroastrianism, or Bhaal and Astaroth are a sufficient pantheon for a people group.
I don't think, however, that that is sufficent for a living, breathing campaign world. The tribe over there should worship one god who has given his people victory in the mountains and on the plains. The tribes over there worship whatever spirit is granting their current leader power. The city state over there worships their emperor as a god and is forbidden to offer prayers to any other god. Another city state worships their exalted ancestors and yet another one doesn't exactly worship anyone but focusses on a strict pursuit of an aescetic philosophy. The empire worships a pantheon that incorporates the tribal gods of its various conquered peoples into a semi-coherent mish-mash with multiple overlapping gods. Exactly which of them are gods proper and which of them are worshipped by clerics (as opposed to druids, shamans, etc) is not exactly clear. They could almost all be fictional, animated by one or two powers for specific purposes, or they could all be real with a godling behind each hill and dale and gods rising and being slain each year.
