GIGABYTE 990FXA-UD3 Rev.4 and rev.5 have 6 PCIE slots and work perfect!!! Used by me with 4 X 7990 and two 970 GTX! All 10 GPU work perfect.
Most would not mix AMD dual gpu cards which each show up as 2 gpus and Nvidia cards which is why you reached 10 in windows 10. The driver for a single type of gpu in windows 10 is 8 gpus.