[G&K] workers

I seem to have a little bit of trouble deciding how many workers that I need for my civilization, particularly in the early game. I've seen a number of videos that feature to workers for the capital alone, but when I've tried that myself, I often end up with workers without enough to do. I've had the opposite problem as well, where I needed more workers than I have produced and only realized it some turns later.

Are there any good guidelines for how many workers you want in any given situation?