BTC-37 GPU Mining Rig Machine Motherboard With CPU support 8 GPU PCIE slots | eBay
The ASUS H370 Mining Master connects 20 GPUs to one motherboard with PCIe over USB - Edge Up
Amazon.com: LGBOY 12 GPU Mining B250 Motherboard Support G3900 CPU LGA1151 DDR4 RAM Maximum 16GB Mainboard for 12 PCIE 1x 16x Graphics Card Support VGA DVI for Miner Dropship with SATA Cable+Switch :
Asus has a motherboard that supports up to 19 GPUs | PC Gamer