What is the cpu on the board?
You need x16 (16 pcie lanes which is determined by cpu spec).
Slot needs to be set to x16 in bios not x8 or x4.
So for most systems 1 is max gpu thst can benefit from this.
i don't see the settings on mb bios for x16.. but i'm on Core i3 3240.. Board does show PCI-E 3.0.. trying many overclock for vram .. max still 18.. hardware limitation? I mean PCI slots.. CPU... or anything else..