Can I upgrade iMac 27inch 5K Display's RAM from 8GB to 32GB/64GB?
Yes, you can upgrade the RAM, and have many different options. I bought the very same system you did, and it came with 2 4 GB chips in the four available slots.  I ordered two 4 GB chips for $100 US from https://www.macsales.com, received them 3 days later, popped them in the two open slots and the system recognized the full 16GB; even though the chip pairs are from different vendors, I had no problem with it.

It is incredibly easy to change the RAM on the iMac and MacSales even has videos to show you how to do it. You can take it all the way up to 64 GB. The best way to see if you need more RAM is to open up Activity Monitor and watch the swap space in the memory tab. Under normal circumstances, the only time swap space is used is when the system needs more RAM than is physically installed. So if you see a lot of swap space being used, that means you are using up all the available RAM.

One note: Unix-based systems like Mac and Linux use far less RAM than Windows, so do not compare the two platforms for RAM capacity needs.