Any modern OS, macOS included, will try as hard as it can to use 100% of all available RAM. Leaving RAM unused is a waste.
Read the above three times, then think again if you need this number displayed on the screen. It is really pointless to know the total amount of RAM used unless you understand how the RAM is being used. The OS will use some tricks to make the overall user experience seem quicker, like speculatively moving data into RAM and swapping out data that has not been used in a while. It will also always try to keep a reserve of unused RAM but it is best not to keep too much of a reserve. This is too complex to show with just a single number.