You can only use 4 GPU's for 3d rendering as that is all current drivers allow for.
As like F@H, cryptology can be done with multiple GPU's, as it uses its cores NOT the GPU itself as a collective.
Technically true the FAH clients lets up to 8 GPUs run but there is software mod or something. What happens when you try to load 5+ GPUs in Windows 7 with pocblm it doesn't load drivers for any card after the 4th GPU and when you assign device=4+ it doesn't recognized it. Core-wise I'm not so sure if Windows recognizes dual-GPU cards as 2 GPUs or 1 GPU. This is from personal experience with 5x5870 so other people might have had better luck.
Again, I said that in theory. Honestly, you will run into scaling problems. When you run <3 GPU's on a single computer, you are losing performance.
2x the same GPU does NOT equal 2x the performance