Why is the "Mac Pro" forum the wrong place to ask a question about a Mac Pro?
But one answer is "No Z-series". There's no Z-series with only single socket and only dual GPU.
And another answer is "All Z-series" - since they'll all be using the same CPUs, chipsets, and memory as the Trashcan Mac Pro. HP lets you choose from 1 or 2 sockets, 1 to 4 GPUs, up to 512 GiB of RAM, quad 10 GbE, ....