http://www.asrock.com/mb/Intel/X99%20WS-E10G/
The best motherboard currently with 7 PCIe 3.0 x16 slots. Can support 4-way sli/crossfire with x16, x16, x16, x16 mode or run x8 at all if all slots populated.
You cannot 'sli' more than 4 cards (at the moment) so it's useless for games etc. Avoid it unless you need single machine High Performance CUDA for cad/rendering/media/number crunching. (stand alone mining ASIC units are much cheaper than these monsters for bit-mining)
There are only two options and you better be sitting down as they are both expensive ($3-5k expensive)
Tyan have a few different configurations for 8-way GPU machines.
if your budget will stretch, you can fit 6 cards in a red harbringer case, and using pci-e extenders on the aforementioned Asus X99-WS board. If you use dual gpu cards (590gtx, 690GTX, Titan-Z) you can get 12 gpu's working together on a budget.