If I/O speed is important the challenge will be getting lots of nvme slots in a small form factor. Many atx motherboards have bifurcated pci-e slots that can be converted to manage 2 nvme drives at once (in addition to on-board nvme slots) but I don’t know if matx boards do that, so if you wanted 3+ drives that would be the first thing to consider. If you just want a bunch of sata ssds there are more options, but all considerably slower.