Nothing special; each Xeon socket can provide 5 PCIe x8 slots. Don't try this in a 1U though.
Thinking about it, that (2 x NVMe in PCIe) will probably be the first upgrade I do to my new workstation in the future. It looks like those only use four lanes each and I've got enough still available (80 total) to support way more of these drives than I expect to ever need on my desk.
ETA: PCIe v3.0 x16 == 15.75 GB/s. I expect we'll see, at some point, PCIe cards/adapters that support up to 4 NVMe drives (in one slot). Hell, they might exist already.