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:
- 255 LUN IDs per target
- 128 targets per bus
- 8 buses per adapter
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.