Q: What is the maximum number of LUNs that Windows Server 2008 R2 supports?

A: Realistically, there's no limit for any possible hardware configuration. If you look at the numbers that would make up a limit, it appears like the following:

So that total gives 255 * 128 * 8, which is 261,120 LUNs per Host Bus Adapter (HBA). I don't think anyone will get anywhere close to that many, and to my knowledge, I don't think more than a few thousand is tested.