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.
I asked Carter once about it..he said 13 regular GPU is the limit and rest will need to be mining GPUs...
I wouldn't mind buying mining GPU's but 1060 equivalent for $600+? Riiiiight.....I'll just buy me 1080's from ebay and it comes out cheaper