The suggested numbers from ASUS website has only 3 configurations for the cards to reach the maximum number.Probably it is not updated but it is a good start to have a look there.The processor of that motherboard is an intel and as far as I know it supports up to gen 9 processor for the LGA 1151.The maximum you will go with a single vendor i.e only NVIDIA is 16 graphic cards because of the best processor i9 9900 supports 16 PCIEx lanes max.
Here is the link the official faq from ASUS :
https://www.asus.com/support/FAQ/1034921I'm curious how Asus was able to get 19 cards to work if most Intel processors only support up to 16 PCIe lanes. If using a variety of vendors is supposed to work, the initial question still remains, how/why?
As for using other processors, this motherboard uses an Intel® B250 Chipset.
Supported processors are stated here:
https://www.asus.com/us/Motherboards-Components/Motherboards/All-series/B250-MINING-EXPERT/ using the Intel LGA1151 package.
Another concern about using the recommended variety of GPUs is that they are becoming obsolete relative to the DAG file for Ethereum mining.
My current processor has a maximum of 16 PCIe lanes:
https://ark.intel.com/content/www/us/en/ark/products/97123/intel-core-i5-7500-processor-6m-cache-up-to-3-80-ghz.htmlAlthough I can only utilize 14 NVIDIA GPUs. I assume the remaining lames are being used by the gigabit LAN port and something else, not exactly sure.