This was marketing from asus since beginning, all other manufactures have only up to 12 or 13 gpus which means they know that is the maximum gpus windows recognizes, i wonder if somebody with linux got 19 gpus to work with this motherboard.
There're some members already posted their successful tests with 19 or even 21 GPUs with B250 in Window 10 Fall Creator version 1709, but the GPUs they used are dedicated mining P106-100. Even me, I also run 16 GPUs with this motherboard. For AMD GPU, of course we could not go over 13 GPUs, I ever saw Bit Be Trippin upload a video which he built 13 AMD (blockchain driver) + 6 GTX 1060.