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#msg24031325If 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.
I'll go with Vann's advice, even you manage all cards to be working on one Rig this is not practical for since it has a centralized failure, mining is not a 100% reliable all the time of the day, if any of the GPU's, PC components, Softwares or even connectivity failed, your profits will greatly reduced.
Also, each RX470 will draw 180-190w in dual mining, 4x650w PSU in 18GPU is not sufficient to power them all