Created on: May 11, 2013
Last updated: February 29, 2016
Unfortuntely there is no one correct answer to this question.
It depends directly on the specific Mac you're using, the operating system you're running on that Mac, and the specific software applictions you run from day to day. While Apple lists only 2GB as the requirement for their recent operating system versions, this is a bare minimum for running only the apps that came with the computer. Even in this scenario, if you run many apps at the same time with only 2GB of RAM, the system responsiveness may slow down.
For that reason, if you want to leave yourself some leeway to use other apps or multiple apps at the same time, 4GB is often good minimum starting point. If you intend to work with digital audio or video on a regular basis or (for example) the Adobe Creative Cloud suite, you will likely need 8GB or more. Please consult your Mac user manual to determine the total amount of RAM which can be installed, and the developer of your favorite apps to determine their RAM requirements.