It would seem to be physically possible to fit dozens of m.2 slots on a motherboard. Why can't I find a motherboard with more than 3? Surely servers would have a use case for more than 3 m.2 ssds?
- The M.2 slot requires and x4 pcie equivalent to have enough bandwidth to function at the expect speed.
- The chipset has a limit to how many pcie lanes are physically available.
- Some of the lanes are dedicated to other devices.
- Physically space requirements as most M.2 lay flat down against the motherboard. each one is approx 22.1 by 80.26 =2.75 square inches if you want a dozen that is 33 square inches. The ATX size clearly doesn't allow for a CPU, and squeezing RAM on there would be tricky at best.
EATX probably has enough room for a CPU and maybe RAM, but nothing else. Since the northbridge chipset requires active cooling there is no room for it under the M.2's.
SSI MEB would have enough room and I don't think those are very common.
WTX would have enough room, but I don't know how common they are.
So screws may secure them into place.
5. You can't place them close to the PCIe slots due to potential pcie card over hang.
Cards like this allow 2 in 1 pcie slot
Silverstone SST-ECM22 Dual M.2 to PCI-E X4 NVMe SSD/SATA 6G Adapter https://www.google.com/aclk?sa=l&ai=DChcSEwiMq7y8hLbeAhWFJmkKHSq4D_4YABAZGgJpcQ&sig=AOD64_3t6MTcwHm1WdEACCQOhozV3m3Kgg&ctype=5&q=&ved=0ahUKEwi-47e8hLbeAhVrw4MKHTKOA2E4ZBDYKQiIAw&adurl=