Asus replies to me today:

"There should be no need for update, this board should support 19 cards already. To run more than 8 AMD cards, you will need AMD Radeon Software Crimson ReLive Edition 17.10.2 driver. Please download this from AMD website. For full 19 cards, for system stability we recommend mix of AMD and NVidia cards, refer to manual page 1-9 onward for details."

So. Hard to say if it is the mb or os that is only capable of 13 gpus.