There is the iPad and iPad 2. There are black and white versions of both. There are WiFi only iPads and WiFi+3G iPads. The iPad 3G and iPad 3G are for Verizon and AT&T. There are 16, 32, and 64 GB models for all of them.
So depending on the information you're looking for, it could be as few as 2 (iPad/iPad2) or as many as 48 (combinations of all of the above.)