I have one of these boards in my setup. The board has no problem recognizing 6 physical cards. There seems to be a quirk with this board I have found that prevents it from recognizing card #7. Seems that the board disables one of the 1x slots when the the adjacent 1x slot is occupied. Found a couple of YouTube vids confirming this issue.
Under that premise, I see no reason why you couldn't load this board up with 12 GPUs. I do not have any way of testing this though since all of my cards are single GPU. I'd love to see pictures of something like that.
I settled with 5 x 7950 cards on mine.
Edited for clarification.
I have heard that 8 GPU limit is imposed by the PCI express design or operating system design. I never seen anybody who run more than 8 GPUs on the same motherboard. Please post a link if you have seen otherwise.