i have did that on maximus v formula with m.2 to pci-e adapter, its succsessfully finds 8 gpu on device manager without error, but somehow i didnt get reliable mining, windows frequently freezing when mining @ 8gpu. for additional info i was connected 8 rx470 4gb
you must use for 8 gpu linux, there is also os optimized for this like ethos (payment) or miningOs (free) or you can try to do all by your self with a ubuntu installation.
Did you use a custum bios for you MB or you use standard bios?