If you need two or three masters on the weight axis depends on the design of the font. So you can try to us only two, interpolate the instances and see if you like the result. If not, you add add the Regular in the middle. If you need corrections only on some glyphs, you can add add extra 'masters' per glyph (in Glyphs.app with a brace layer and with an extra UFO in designspace files, I believe). That way you save a lot work.
I have no expertise in striking punches with a hammer. But I once tried to get a 20cm nail into a log with similar hammer as seen in the image above. I nocked on it dozen times and it move it around a 1 cm. Then my cousin tock the hammer and stroke three times and the nail was in. What I’m trying to say is that the number of strikes and the wight of the hammer you need to punch can vary VERY much by whom is punching