How much water/sugar/yeast does it take to make how much ethanol?

It is usually measured by specific gravity. The more sugar the solution has the thicker it is and the higher its specific gravity. Yeast can only ferment to 13-15% ABV depending on strain. So the maximum amount of sugar you could use about 3 lbs of sugar in one gallon of water to give you a specific gravity of about 1.100. And yes it is very close to simple syrup. Of course most people actually ferment things like fruits or grains, not pain sugar. But if you let that ferment to completion (when its specific gravity is 1.000 you would have an ABV of about 13.5%.