Haha after 3 days struggling I finally achieve the impossible, 6x1080ti on a AM4 motherboard mining guarantee, not counting 2x M.2 slots. I could achieve 8xGPU if i have enough part which are on the way shipping.
Spec: Ryzen 7 1700, Asrock X370 Killer SLI/ac (2xPCIe 16X + 4xPCIe 1X + 2 SATA/PCIe NGFF M.2). Windows 10
Riser: 6x Ubit riser latest version with LED. One came defective, contacting them for the replacement.
MSI Gaming Pro/Gaming Carbon Pro will do 5 / 6 cards without many issues. Update to Fall Creators Update, have DDU handy if something goes wrong the first time. I found it was pretty easy to get 6 Nvidia cards working on these AMD4 motherboards. It was trickier with the AMD Vega 56's, but Fall Creators Update and other tweaks solved that as well. The nice thing with these Ryzen builds is that you can have several threads of the CPU working away on Monero while the GPUs mine ZEC or something else. Easier to pull off on Ryzen motherboards than with i3/i5 Intel builds.