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
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.
WHooooaaaah! That is a BEAST! Cool! not profitable, but Cool!