"Intel Core 2" is the brandname. 2 is simply because this family of processors succeeds the "Intel Core" (1) family. It includes Solo (1 core), Duo (2 cores) and Quad(4 cores).
That nomenclature Intel uses is pretty confusing for some. Many times I had to explain to people that just because it's marked Core2, it doesn't mean it's Dual core. In Intel's case "2" means Second generation only.