Crucial is correct. You can go all the way to 4 TB, if you want. Dell's maximum memory and storage specifications are published at the time of system design, based on what they tested. It's not a limitation, so don't take it as gospel. Technology evolves, and often larger RAM sticks and larger drives become available after Dell publishes the specs, and Dell does not generally go back to revise the maximum specs. So, Dell's specs should not be interpreted as limitations or maximums.
Dell says the max SSD my inspiron 7580 can use is 512G but Crucial says 1 and 2 terabyte SSDs are compatible. Dell is only selling the 512G SSD upgrade for my laptop. Who is correct? Am I limited to 3x4 (gen 3 I assume) SSDs? Specs say I have this currently in 256G: NVMe PCIe M.2 2280 Internal SSD High Performance Solid State Drive
inspiron 7580 laptop SSD size