You can go further. There are relatively cheap adapter boards which let you stick 4 M.2 drives in a single PCIe x16 slot; you can usually configure a x16 slot

You can go further. There are relatively cheap adapter boards which let you stick 4 M.2 drives in a single PCIe x16 slot; you can usually configure a x16 slot to be bifurcated (quadfurcated) as 4 x (x4). To pick a motherboard at quasi-random: Tyan HX S8050. Two M.2 on the motherboard. 20 M.2 drives in quadfurcated adapter cards in the 5 PCIe x16 slots And you can connect another 6 NVMe x4 devices to the MCIO ports. You might also be able to hook up another 2 to the SFF-8643 connectors. This gives you a grand total of 28-30 x4 NVME devices on one not particularly exotic motherboard, using most of the 128 regular PCIe lanes available from the CPU socket.