I've just spent a month trying to get more than 3 graphics cards to run on my Asus Z170 Pro Gaming motherboard and finally succeeded yesterday after much fault finding. Here is what I've found.
1. Download the latest motherboard BIOS to a USB stick and install.
2. XMP memory. I have 2x DDR4 3000MHz memory modules that by default run at 2100MHz. When slotting these in to A1 and B1 with more than 3 graphics cards and enable XMP, the system failed to POST. So pop these in to memory slots A2 and B2 and enable XMP.
3. Access the BIOS and navigate to Advanced>System Agent (SA) configuration. Enable Above 4G Decoding.
4. Access PEG Port Configuration. I have my main graphics card in PCIEX16_1 and so left that at Auto. However I have a PCI-E 1x adapter in the PCIEX16_2 slot and so I set this to Gen2.
5. Press Escape back to the Advanced main menu and select PCH Configuration and then select PCI Express Configuration. Change PCIe Speed to Gen2. This will enable ALL PCI-E risers to work on the motherboard. If I had known these 2 steps a month ago, I would have more hair.
6. I've found it important to not just switch off the motherboard, but to turn off the power supply (PSU) each time I unplug/plug in anything. If your PSU doesn't have a switch then unplug from the wall.
7. Switch off the power and install a single graphics card in your mining rack closest to you. We're fault finding Risers here. Connect your monitor cable to the card.
8. Fix a riser to the graphics card and plug in the power cables. Plug the 1x adapter in to any PCI-E port on the motherboard.
9. Turn on the power and see if your computer POST's. Hit Delete because you don't want to boot an OS just yet.
10. If your computer POST's then switch off (and turn off the power supply switch), unplug the 1x adapter from the USB cable and keep it in the "good" pile. Do this with all your remaining 1x adapters.
11. If your computer fails to POST on the first riser, then switch off and try the next.
12. Once you've checked all the 1x adapters, then switch out all the 16x risers to see if they each boot. All my six 16x risers worked but two of the 1x adapters were faulty. I already had backups but I'm getting the faulty adapters replaced from the supplier.
13. Switch off and install one card at a time in your Operating System - install a card, boot to your OS, install the drivers, mine something easy like Ethereum for a few minutes. If all goes well then switch off and install your second card etc...
14. Do this for your remaining graphics cards and finally you should have a nice mining rig set up just like mine
I hope this will help others with most Z170 chipset motherboards too.