Re: 12 GPU , 13 GPU mining motherboards is now available
Windows can take only 8 gpu so 8 slots are enought 
You can share the AMD and nVidia cards so the total is more than 8.
Yes, Windows can only handle 8 GPU's of ONE TYPE of card. Theoretically, you could have 8 AMD and 8 NVIDIA mining under windows, but I imagine that would be problematic and unstable.
With a new 12 GPU board, 6 AMD and 6 NVIDIA in windows works fine. There are several videos and forum posts I've seen with people doing this.