What if use 7 such multiplerlers with 3 cards in each to build 21-card rig on a single motherboard? What if install 3 multiplers in each multipler resulting in 63 card per rig? Where significant performance drop will appear?
Look at the posting by citronick. Looks like there are other issues that come into play.
It has been done actually by Nippy -- see this link and see the motherboard (its a USD$2000 PCI extension board for deep learning systems, like Facebook's Project Big Sur)
https://bitcointalk.org/index.php?topic=1433925.msg16380747#msg16380747
https://www.sabrepc.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/1/8/1895.jpg
Its not as straight forward as ... just putting these multiplexors into an array ... you need Linux and special AMD drivers to make this work.... its entirely different animal all together. Claymore even needed to put a new version of ETH miner to support that many cards.
Actually big sur while it does have an expansion board it only uses 8 gpus, although it can run k80s which would then need changes similar to the OSS board, it would still not need much in the way of bios mods to work, rather a small hypervisor kernel change. The stock linux kernel is not happy with OSS boards iirc, and they do have a hard limit of 16 cards(just in the way of how graphics cards are allocated space today). If you want to see an example bios for a system that was made to work with 8+ cards on an amd mobo you should look up fastra II
