The manual references 40 lane cpu and 28 lane cpu...but either should provide more than enough lanes for 6 cards on risers. What i dont see is above 4g decoding in the bios. That should limit you to 4gpu...but you should easily get 4 running on risers and turn off everything you dont need...audio/m.2 ports / sata (if using thumbdrive and moning os)....VTD off .... pretty much anything not needed...so sleep mode on proc or energy saving.
Ive used the 1-4 pcie multiplexer with risers to add 6 gpus to a board with a single x16 slot and 2 x1 slots. So they work. Ive got much more expensive ones that you can actually cascade..1-4 to 3 gpus and another 1-4.... but pcie lanes and bios/chipsets will eventually limit you.
ok i see, i had 3x GPUs running on this board years ago and it work fine and fully supported, the only bad thing is that u need to insert the GPUs in Certain PCI-E Slots, if u try insert them in ANOTHER PCI-E Slots they wont work !
i have enabled 4g decoding for this board, ill try plugging through riser, but as i said this board supported 3x GPUs before but now when i insert the third GPU in another PCI-E Not recommended the board will simply NOT see it !...