Open Activity Monitor (in Applications>Utilities) and click on the Memory tab. Then start monitoring the Memory Pressure graph while you do whatever you fairly regularly do (or plan to do).
1 - If the graph is already (or quickly goes) yellow or red - you need a memory upgrade.
2 - How quickly it goes yellow/red as you open apps will give you a very, very rough estimate of how much memory you need.
3 - You can improve this memory upgrade estimate by running each regularly used application separately and seeing how much memory it uses “all by it’s lonesome”. Then total up the app-memory usage by each regularly used app.
FWIW on my 2014 Mini with only 8GB memory I had to religiously monitor my memory pressure and quit applications to keep the memory pressure graph green/yellow. Now with my 2018 Mini w/32GB memory — I don’t even think about it
As Steven.R & Ybersetzer posted -- You definitely want to use two matched DIMMS.