There aren't that many motherboards available that can run more than six video cards, but there are few like Asus Prime Z270-A for example. This particular motherboard has 7x PCI-E slots and two M.2 slots for SSDs that can be converted to PCI-E connectors for additional video cards.
RE: 8x RX 580 GPU Crypto Mining Rig with Asus Prime Z270-A Motherboard