I have the same MOBO and ordered an M.2 to pci-express adapter.
It will go as the 7th GPU, so you can use 7 GPUs for sure, but I don't see any problem getting a splitter to push 8 GPUs.
I just have the PSU for 7 GPUs max.
What are your settings to get 7 GPU:s working? I can not get the motherboard to detect more than 6 GPU:s