The Synology DS1623xs+ tops out at six SATA bays with NVMe caching

Six drive bays give the DS1623xs+ serious capacity for a compact NAS. The two M.2 NVMe slots provide SSD caching only — they won’t appear in your storage pool. If you’re populating this with six 3.5-inch drives, the cache will help, but know what those slots are for.