Hello,
I have the board on 9 rigs. It is working but i think its a little sensitive when you run with 6 gpu.
 I tried 6 gpu only on simplemining OS and its working, not maybe from 1st shot, but after switching GPU-s on mobo, or changing some usb's from risers, it does work.

Technically, you can install 7 gpu's, it has an available m.2.
Cheers