You can run 12 AMD RX or 12 Nvidia cards easy under windows , and if you know what you doing how to set up your rig properly , you not going to have any problem !
if you want more than 12-13 cards , you must to mix nVidia / AMD / And mining cards
With ASUS B250 MINING EXPERT its possible to run more than 19 GPU's under windows , but you have to mix nVidia / AMD / And mining cards .
12 AMD Radeon RX 470 on one rig + 8 Mining cards
thats 20 under windows13x1080TI Windows 10 - 1709
13 AMD Radeon RX 470 on one rig + 8 Mining cards
thats 21 under windows 