I'm using this motherboard with two 1-to-4 PCI-E multipliers put in the x1 slots for 12 cards in total. Works flawlessly with no additional bios tweaks besides enabling mining mode. You must convert your windows install to GPT in order to avoid error code 43, though. Linux should work out of the box with 6+ cards.
I'm getting BSODs after connecting 9th GPU with the same hardware on win10 1903 and 1703 and "dag fix" driver..
Though after the BSOD and auto-reboot Win10 somehow loads.. with 8 GPUs visible, probably some bypass feature. From the info I've dug yet, I'm not sure whose issue is this..
Thus, can you please tell your windows version and build (from 'winver' command) and video driver version and which exactly slots are the mults in?