On Intel-based motherboards, when you exceed 4 GPUs (regardless of brand, and older or newer GPU), the PC Bios throws 8 beeps and error D4 "PCI resource allocation error, out of resources." This happens before O/S is started, so the O/S doesn't matter. The solution for this is change settings in the PC Bios to 1) Disable CSM support (Compatibility Support Module, for legacy bios compatibility), 2) Enable "Above 4G decoding", and 3) Enable "Secure boot" (drive must be formatted GPT, not MBR, you will have to reinstall the O/S if this one needs to be changed). These lessons all learned the hard way.
A unique problem to the Radeon RX 6xxx (regardless of manufacturer) is this: Whether there is only one GPU, or more than one, on an Intel-based motherboard, the PC Bios hangs at step 92 "PCI Bus initialization is started." Again this happens before the O/S is started, so the O/S doesn't matter. The solution for this found here https://itigic.com/error-92-nvram-on-amd-rx-6800-how-to-fix-it/ was to ENABLE CSM support in the PC bios. I verified, that works. But note in paragraph above, enabling CSM Support limits me to only 4 GPUs. So, if I go with all RX 5700 XTs, I can have as many GPUs as my O/S and motherboard will support, but if I want EVEN ONE rx 6800, I am limited to 4 GPUs per PC, period. Bummer. (Apparently this is not an issue on an AMD motherboard.)
I know a lot of people hate cryptominer hobbyists and video renderers and such, but we do buy an awful lot of GPUs. Sapphire Support directed me to tell AMD about this, since it is not a problem with their brand. So here I am. I AM HOPEFUL THAT AMD FIXES WHATEVER IS WRONG WITH THEIR RX 6xxx ON AN INTEL MOTHERBOARD so that CSM Legacy Support is not required for their GPU to pass muster with the PC Bios. Perhaps AMD never tested multiple GPUs in an Intel motherboard and aren't even aware of this problem. This is NOT a problem with the RX 5700 XTs, where I can run 7 GPUs per PC.
Otherwise, I love the new AMD GPUs, and I would look forward to buying more.
Motherboard: X299-WU8 (intel based) - GPU: Sapphire Pulse RX 6800 - RAM: 32 Gig - O/S: Windows 10 Pro
