How many extension tubes needed?
It depends on how close you want to get. In theory 50mm of extension tubes will give 1:1 on a 50mm lens at infinity, plus how close the lens focuses natively. Many lenses shorten their focal length at their closest focus, and therefore the extension tubes will have a greater effect than indicated by nominal values.
With 37mm of extension on a normal 50mm lens you should be able to get to just below life-size using the close-focusing of the lens itself. With the Canon compact EF 50mm f2.5 macro you would get over life-size. So the question is whether near to life-size is close enough for you. If it is you don't need more extension tubes, whereas you might need more extension if you want to get closer than that.