Highpoint and Squid are your goto options in that space. There are others, e.g. dumb cards, if you just want to bifurcate an existing PCIe slot to handle four M.2 drives. But if you want 16 or 64 NVMe drives, the two manufacturers I mentioned have offerings. I have a Highpoint U.2 card, which gives me the option to handle 8x U.2 or M.2 drives (with adapters) in a single PCIe slot.