Some motherboards have been reported to do 8 GPUs with an addon board. The MSI Z170A GAMING M5 is one of them. There is a thread in this section detailing how to get it to work with 7 GPUs; I'm not sure if there is anything extra needed to get an 8'th GPU to work or not. We will be building 2 rigs with that motherboard sometime in May, so I may try to get a 1 to 3 pci-e riser board to test at some point.