8 GPU:
ASUS Prime Z270-A
ASUS Prime Z270-P
ASUS Prime H270-PLUS
IMHO, any mobo (h270 or z270) with 6 pci-e + 2 m2 can support 8 gpu
i see a video that say up to 10 gpu can be supported and under linux a crazy 12 gpu, this is like having two rig in one very convenient and saving money, here
https://www.youtube.com/watch?v=UUM2GcidY08