How many volts until it is harmful?

That is difficult to answer because there is no fixed answer. As an experiment put both terminals of a 9V battery on you tongue. It will not hurt you but you will surely feel it. Now put your thumb across the terminals of that same battery. Two things should now be obvious to you: 1) that even low voltages can "hurt" you under the right conditions, and 2) voltage is not the only factor that determines how harmful.