I got 5 up and running on it. Tried to do 6 but something with the dual PSU setup was screwing up the output. Either that or shorting the very top 1x bus confuses the motherboard as it expects to send video from the 16x main bus. You could maybe do 6 if you skipped that top 1x bus. I didn't fool around with it too long though as I figured I'm already maxing out a 1600w GPU with 5 cards. I got another 5 GPUs here that I'm gonna do the same with. One tip for this board, you have to set the timings for the PCI-E busses manually in the UEFI in this order: 2nd gen, 1st gen, 1st gen under chipset configuration. This is the configuration for 3-way Crossfire/SLI. I was pulling my hair out trying to figure this out because anytime I'd add more than 1 card to the thing it wouldn't give me any output, yet it claimed to be able to handle 3x Crossfire/SLI. A quick peek through the manual showed the correct bus speeds for the 3-way. Even though we aren't running the cards this way, you need to trick the board into thinking that you are. Having them set to auto doesn't work. Other than that this is a great board. I just bought another one to set up the rest of my GPUs with. Shorting the pins is easy if you use twisted pair from inside a CAT5.

For a first build 5x GPUs is plenty. I was like you and wanted to go all out, planned to run 6 off this system with dual PSUs, but over the past month of experimenting with hardware I've found that 5 GPU + 1600w PSU is the perfect setup. After 5 GPUs it starts to get unstable and finicky. Might as well just build another rig at that point.

Hope this helps.