How much RAM memory do I need for an OSX device?
I have the Mac Mini 2018, and I bought it with the 'default' 8GB of RAM. With several apps open, but just idling, it was at the 'top' of the Green memory pressure band, and loading it beyond that, either with 'work' or data, would start to bring it into the yellow band. 

I regularly had about 2Gb of swap, and the same of compressed memory. Don't forget that the Intel Graphics unit takes 1.5 Gb straight away.

I've just installed 32 Gb of RAM (from Crucial, not Apple!), and the difference is palpable.