I recently started to work on a build with a Asus B250 Mining Expert motherboard. With no intention of putting all 19 gpus in it any time soon but maybe in the future. I was aiming to just add 1 card a month until I had 6 then give it some time. I got it up and running pretty quickly with 2 graphics card with windows after spending hours and hours with linux.
I decided to pick up a Rx560, Rx570 and Rx580 so I would have an opporunity to work with all 3 cards and better decide what to use in my builds. As of this writing I was only able to get the Rx570 and Rx580 working together. The motherboard just will not accpet a 3rd card into the rig. Trying for an entire day which moving wires and setting with nothing working. The only cause I have been able to come up with it that even though they are all 500 series card you are still limited to just using 2 different cards. The builds that I have found online all show builds with a maximum of two different card. It seems if you wanted to get to the 11x AMD cards in the system you would either need 11x of the same card. In order ot get to the 19x gpu limit some combination of 11xRX5xx + 8x P106 mining cards would be needed currently.
Unless your buying the B250 Mining expert with the hopes of windows/card drivers being update to more easily get the system up and running with 19x gpus I would suggest you purchase a motherboard for the 6-7 gpu limit area. This will save you alot of headache as from my understanding every card you add to the B250 mining expert will increase you troubleshooting time to get it up and running by more and more each time. Then once you hitting the 11x gpu + you could be spending an hour trying to do something that when you had 6-7 cards would take your a couple minutes.
Some suggest boards for rigs are listed below-
MSI Pro Series Intel Z270 DDR4 USB 3 CrossFire ATX Motherboard (Z270-A PRO)
MSI Arsenal Gaming Intel Kaby Lake Z270M DDR4 HDMI USB 3 CrossFire ATX Motherboard (Z270 GAMING PLUS)
MSI Enthusiastic Gaming Intel Z270 DDR4 VR Ready HDMI USB 3 ATX Motherboard (Z270 GAMING M3)
MSI Enthusiastic Gaming Intel Z270 DDR4 VR Ready HDMI USB 3 ATX Motherboard (Z270 GAMING M7)
MSI Performance Gaming Intel Z270 DDR4 HDMI USB 3 SLI ATX Motherboard (Z270 GAMING PRO CARBON)