Four PCIe x16 graphics cards can be used in one PCIe x1 slot.
"The PCI Express riser card from Delock can be plugged into any PCI Express x1 slot on the mainboard. The PCI Express board can be mounted in a different position in the enclosure. This allows up to four graphics cards to operate in a single PCIe slot on a mainboard, even if the graphics cards are wider than a single slot."