NiceHash have published some interesting experiments they have done recently, building an 8x GPU mining rig using the Asus PRIME Z270-A motherboard. This motherboard has 7 PCI-E slots as well as two M.2 SSD slots and they have utilized the 7 available PCI-E slots and one of the M.2 slots with an adapter to get 8 video cards working. Their experiment was with 8x Nvidia GeForce GTX 1060 on that particular motherboard from Asus, though other cards will most likely work as well just fine. With the recent price spike and unavailability of cheaper 6x GPU motherboards for mining people are already looking into more expensive alternatives that work well for six or even more GPUs like this model from Asus for example.