Hello [NAME] , Dell has tested and validated your system with max of 32 GB of RAM.  It has two slots so if you have 2 x 8 GB, you will have to purchase a whole new kit of 2 x 16 GB.  If you have only 1 x 16 GB, then just need to get one more of 16 GB.  You will need DDR4-2666, 1.2V, and CAS = 19.  You can find your compatible RAM here. While some users have succeed in upgrade this system to 64 GB of RAM, your 8th gen. processor may not supported such upgrade and it also reduced the speed to 2400mhz.  
Hello everyone, I have 8geh i7 and I got my laptop with 16gb.  I don't know if it's 2400mhz, or how much, but it's working pretty nice. Furthermore, I am not a gamer, but I have a read that upgrading the laptop to 32 or even 64 GB of RAM can be very helpful for browser and videos load up a bit faster and that the improvement is noticeable. My question is, do I need the same RAM kit, or I can mix? If I buy new set, it means I need to buy 2  16 GB each one and if I can mix it means I only need to add another 16 GB RAM.  I was looking for an answer but couldn't find, all the places says it varies by the motherboard and a few more factors. So maybe if someone is familiar with the INSPIRON and can tell me if I can mix?   Thanks guys
Is It OK to Mix RAM Kits? Inspiron 7590