motherboard with 5 pcie x16 slots

ASUS mining motherboard features 19 PCI Express® (PCIe®) slots. With 18 PCIe x1 and one PCIe x16 slots on board, plus three zoned ATX12V power connectors, B250 ...

Compatibility: Ensure that the graphics card is compatible with the Z370 motherboard's PCIe slot. The Z370 motherboard supports PCIe 3.0 x16 slots, which offer higher bandwidth for better graphics performance.

Another scenario of having four 4.0 lanes or two 2.0 lanes is conservation of PCIe 5 lanes in certain motherboard chipsets...for instance a mobo with 20 max PCIe 5 lanes could then use x16 in the GPU slot and two x2 SSD drives.