The mining craze now also if effecting motherboards, partners, ASUS for example just presented a motherboard based on the Intel B250 chipset. This motherboard comes with 19 pci e-slots. ASUS Mentions it you may combine eight cards,  both Nvidia and AMD for a total of sixteen (with the help of risers of course).

The interesting thing that I can think about it that AMD/Nvidia drivers can support indeed only up-to 8 cards of each, ergo you must use the two vendors. There is word out that AMD however is working on a new driver that will allow for support up-to 11 cards. Then add 8 from Nvidia and you'll have utilized at 19 slots. All slots are pci-e 3.0 x1, aside from one pci-e 3.0 x16 slot.

The board shows three 24pins 12v jacks to provide power to the pci e-slots and GPUs. ASUS also mentions that the unit has a dedicated mining bios, for optimized hash rates. The Intel B250 chipset based motherboard furthermore has two ddr4 memory DIMM slots and four sata connections, alongside a Gigabit Ethernet jack and Realtek audio.

The board should be available soon with a price TBA.

Asus Demos B250 Mining Expert motherboard with 19 pci-e-slots