The difficulty with pcie to m.2 adapters is you usually can't use bifurcation below x4 and active PCIe switches got very expensive after PCIe 3.0.

Used multiport SATA HBA cards are inexpensive on eBay. Multiport nvme cards are either passive for bifurcation and give you 4x x4 for an x16 slot or are active and very expensive.

I don't see how you get to 16 m.2 devices on a consumer socket without lots of expense.