I don't know if there are boards that implement this, though, I'm only looking at systems with 4x GPUs currently. Even just plugging in a 5kW GPU server in my apartment would be a bit of a challenge. With 4x 4090, the max load would be below 3kW, so a single 240V plug can handle it no issue.
https://c-payne.com/products/pcie-gen4-switch-backplane-4-x1...
Not sure if there exists an 8-way PCIE Gen 5 Multiplexer that doesn't cost ludicrous amounts of cash. Ludicrous being a highly subjective and relative term of course.
They're the stuff that enables cloud operators to pool like 30 GPUs across like 10 CPU sockets while letting you virtually hot-plug them to fit demand. Or when you want to make a SAN with real NVMe-over-PCIe. Far cheaper than normal networking switches with similar ports (assuming hosts doing just x4 bifurcation, it's very comparable to a 50G Ethernet port. The above chip thus matches a 24 port 50G Ethernet switch. Trading reach for only needing retimers, not full NICs, in each connected host. Easily better for HPC clusters up to about 200 kW made from dense compute nodes.), but sadly still lacking affordable COTS parts that don't require soldering or contacting sales for pricing (the only COTS with list prices seem to be Broadcom's reference designs, for prices befitting an evaluation kit, not a Beowulf cluster).
nit: HN formatting messed up your math in the second sentence, I believe you italicized on accident using * for equations.
That’s the limit of single CPU platforms.