Another entry in the race for more Pci-e slots the Asus B250 competes against similar boards like the AsRock H110 Pro BTC+ and the Biostar TB250-BTC PRO which only have 12 and 13 slots. It looks like you can only use 16 slots without going with Linux as you are limited to 8 gpus of a kind (AMD, Nvidia) with Windows.
http://cryptomining-blog.com/9013-asus-b250-mining-expert-motherboard-with-19-pci-e-slots/
The new Asus board has 3 atx 24 pin power connectors! and 4, 4 pin molex although you will be using self powered risers in which case you dont need the 4 pin connectors on the board. Its going to be a challenge to space 19 gpus with risers, i expect to see some interesting rig designs popping up as these boards start reaching the public.
Happy mining! (i love this gif)
We wish the best in waiting