The ATI divers dO not support more than 8 GPU's per machine
Yep. Though he did not claim to run that many (he said it's a display bug), ATI/AMD does not support this.
Nvidia does support ~16 gpus for parallel processing tasks like SETI@home
but even then you need 8 dual gpus (& nobody mines bitcoins on nvidia cards seriously),
and there is maybe 1 motherboard that even has that many pci-e slots (MSI BB Marshal)