This article is just looking at the Mac Pro with two AMD Radeon Pro VEGA II Duo MPX modules.

Apple says the four GPU combo is capable of driving:

  1. Six Pro Display XDRs or 6K displays with resolutions of 6016 x 3384 at 60Hz
  2. Six 5K displays with resolutions of 5120 x 2880 at 60Hz
  3. Twelve 4K displays with resolutions of 3840 x 2160 at 60Hz

If Apple's Thunderbolt routing had taken into account all of the display capabilities of these GPUs and had utilised the 128 PCIe 4.0 lanes from an AMD Epyc 7002-series processor, the four GPU combo would be capable of driving:

  1. Sixteen 4K displays with resolutions of 3840 x 2160 at 60Hz

Regardless of the last paragraph, the six 5K and 6K displays part is wrong.

Apple's four GPU combo is capable of driving:

  1. Eight Pro Display XDRs or 6K displays with resolutions of 6016 x 3384 at 60Hz
  2. Eight 5K displays with resolutions of 5120 x 2880 at 60Hz

I suggest connecting 5K and 6K displays directly to the GPU and not routing through the Mac Pro; however, one could argue offloading and routing through the Mac Pro could be beneficial for some workflows.

Resigning the HDMI 2.0 port to AR/VR is your best bet in a eight Pro Display XDR setup.

I would like to try running eight Pro Display XDRs with two 4K AR/VR headsets without mirroring; I wonder if it would work.