2 socket will let you connect an NVMe SSD for all your high-speed low ... Proper places like Digikey and Mouser will have M.2 connectors nicely sorted by key, height and mounting type.

So that the NVMe M.2 units use whatever they are assigned ... SATA 6G (SATA 3), the latest revision of your SATA storage unit connectors, will increase the bandwidth on the SATA controller ...