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.