Just trying to understand your requirements a bit here Martin. A NAS is a NETWORK attached storage. That is not normally going to be installed inside your PC. The are PCI addon cards that can support multiple M.2 drives. (They may or may not also support NVMe which is an extra option, but as NVMe is basically PCI, and if multiple of them are on one card, they’re not necessarily going to use the full bandwidth, depending on the card.)
If you really do want something with a network connection, and knowing how @Leo (and myself) recommend Synology, I might recommend this to you, but it doesn’t use M.2 for the drives: DS419slim | Synology Inc.