Are two CPU's required for a 6-8 GPU rig?

[dontknowmuch]

Yes and No. In theory 2 CPUs provide more PCI-E lanes so more GPUs can run with proper bandwidth. But they are not required assuming the board you choose has a backplane with PLX chips on it, which switch the pci-e lanes to the GPUs in such a way that it can run several GPUs with fewer PCI-E lanes. I've heard of as high as 32 cards connected to a single 2x CPU board. And at least 18 on a single CPU board.

that is fantastic information.
is there any way of knowing if the board has a backplane with PLX chips? all i know is that the server will be a ESC8000 G4 Intel 2*Socket P(LGA 3647)CP 8*2.5 4U.

thank you