Another option might be to take a motherboard that support PCIe bifurcation, you would then be able to plug more than 1 GPU on one slot. I checked if my Asrock could do that but it can't, there other smaller models from Asrock that can.
With this type of hardware:
PCIe to 8 PCIe you could potentially do some crazy stuff
While I know you can, not really my style. I like to run 5 or 6 GPUs per board, mostly due to electrical wattage issues with my house, so if one rig stops for any reason, the others are still working away. I like to keep the GPU support cost as reasonable as possible: MB, CPU, Memory, and PSU run about $350 dollars. That's $58,34 per GPU based on 6 GPUs. Adding more GPUs doesn't really cut the support costs enough to justify the risk and problems - at least in my mind.