Questions about memory for my 27" iMac (late 2015)

I have a late 2015 5K Retina 27" iMac, have 20 GB (4) 4 GB DIMMS installed and am getting some performance issues after installing Illustrator and Photoshop CC 2018, as well as my clipboard not being able to copy or paste objects, and other functions that cease working after a while in my 3D program. I have to restart the app to resolve the issue. I'm under the impression I need more memory. According to the memory specs I get when I bring up "About this mac", it says I can only put a maximum of 32 GB in. I've seen higher numbers. Which is correct? Also, do I have to pair my DIMMS?

Thanks!

Carl