I've noticed there aren't a lot of reasonable home/sb m.2 NVME NAS options for main boards and enclosures.
SATA SSD still seems like the way you have to go for a 5 to 8 drive system (boot disk + 4+ raid6).
replies(5):
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.
In practice you can put 4 drives in the x16 slot intended for a GPU, 1 drive each in any remaining PCIe slots, plus whatever is available onboard. 8 should be doable, but I doubt you can go beyond 12.
I know there are some $2000 PCIe cards with onboard switches so you can stick 8 NVMe drives on there - even with an x1 upstream connection - but at that point you're better off going for a Threadripper board.
Even that gives you one m.2 slot, and 8/8/8/16 on the x16 slots, if you have the right cpu. Assuming those are can all bifurcate down to x4 (which is most common), that gets you 10 m.2 slots out of the 40 lanes. That's more than you'd get on a modern desktop board, but it's not 16 either.
For home use, you're in a tricky spot; can't get it in one box, so horizontal scaling seems like a good avenue. But in order to do horizontal scaling, you probably need high speed networking, and if you take lanes for that, you don't have many lanes left for storage. Anyway, I don't think there's much simple software to scale out storage over multiple nodes; there's stuff out there, but it's not simple and it's not really targeted towards a small node count. But, if you don't really need high speed, a big array of spinning disks is still approachable.