I don't think there are any consumer boards which support this?

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.