Here's what Dell's specifications say for your laptop:   You may ignore the 1 TB figure. Yes, you may use a 2 TB M.2 drive for your C: drive. In fact, you may go all the way up to 4 TB if you wish. The second M.2 drive may also be up to 4 TB. You may also be able to add a 2.5" SATA SSD, but of course it would be much slower than the NVMe drives. However, if you have the optional 6-cell battery rather than the 4-cell battery, it might block the space where the 2.5" drive would go. (That's the case on many laptops, but I'm uncertain about your model. You would be able to tell when you open the case.) Note that if the laptop was originally ordered and delivered with a single M.2 drive, there will not be a heat sink cover for the second drive. You will need to buy that separately. They are available on Amazon and elsewhere. Follow the disassembly and reassembly steps in Dell's Service Manual. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.  
I was trying to see what drives my laptop is able to take. I wanted to know if my C drive was able to take a maximum of 2TB by samsung, and i was also looking to see if the 2 extra SSD slots compatibility for samsung brands and what their maximum storage capacity can be.
15 R3, compatible SSD for boot drive C and SSD for 2 empty slots