How much memory a system "should" have is a bit subjective. I would argue that a budget system or a pre built these days should have at least 4GB of RAM open when at idle, assuming it has 8GB total. This may vary depending on many factors.
This could vary significantly based on amount of memory and whether you have integrated graphics.
As far as "used" memory I typically see around 2GB being used, in idle with nothing but Windows defender running. On a machine I have with integrated graphics, it's using almost 3
On my actual "in use" machine with aftermarket AV and other utilities running around 4