So after some more reading, I'm now rather considering the Gigabyte B250 instead of the Asus B250 Mining Expert, because shared PCIe lanes are just no good idea.
So what's the catch to be running more than 12 GPUs on the Asus B250? You have to be running GPUs from Asus for the remaining 7 slots, right? If doing it with 12 x GeForce 1070 + 7 x Asus, can it be really running stable?
Running 13 radeon 570. No problemo. Havent tried with 14.
For the last 6 u need mining cards.
My rig 2 is running 10 Nvidia Cards but will show red on the 11th card if added. Im running windows 10 pro updated to 1709. Bios is up to date. 8gb (4gb in each slot). Runs flawlessly with 10 cards and its a set it and forget rig until you stick that 11th card in then it will not see it in device manager. So i obviously didnt follow the owners manual about mixing in AMD cards (I only own 8 AMD cards and they are on a nother B250 m/b). Has anyone else had this no 11th card recognize problem?