Adapting a PCIe slot to a single M.2 slot should always work, so long as the SSD is actually using the PCIe lanes and not the SATA port that M.2 can also support.

Depending on how old the motherboard, or UEFI, is - it may not be able to directly boot from NVMe. Years ago I modified the UEFI on my Haswell-era board to add a DXE to support NVMe boot. You shouldn’t need to do that to see the NVMe device within your OS though.

As the other reply mentioned, if you want to run multiple SSDs on a cheap adapter your platform needs to support bifurcation, but if it doesn’t support bifurcation hope is not all lost. PCIe switches have become somewhat cheaper - you can find cards based on the PEX8747 for relatively little under names like PE3162-4IL. The caveat here is that you’re limited to PCIe 3.0, switches for 4.0+ are still very expensive.