Just one M.2 slot is already too much. You know where they always put it? Sandwiched between the CPU (which almost always means a huge heat sink) and a full-length PCIe slot (which is almost always occupied by a GPU). It’s a challenging topography. I dread having to swap out M.2 SSDs or vacuum dust from the crevices. God forbid I drop a screw while working in that area.
Sure, there are M.2 slots on other parts of the motherboard, but they are usually not CPU-connected PCIe lanes or they share lanes with another slot. That first slot’s lanes are exclusive to the first slot and can’t be accessed any other way.
My solution is to not use M.2 slots as they were intended, but to attach OCuLink or MCIO adapters to each and every one that comes with the motherboard so I can connect devices of my choice to them at the other end of the cables.