You won't be able to run all 18 cards on the same motherboard. Running more than 13 GPU's with the Asus B250 mining expert REQUIRES using Nvidia P106 mining cards.

https://bitcointalk.org/index.php?topic=2357461.msg24031325#msg24031325

If you want to run all AMD cards I would suggest breaking it up in to a 12 card build on a ASRock H110 Pro BTC+ and a 6 card build on a Biostar TB85 or another H110 Pro BTC+ motherboard if you plan to expand.