Not sure that trying to make a wine at 20% makes good sense but that is entirely your business. You would want to aim for a gravity of about 1.150. So it doesn't matter what the starting gravity of your grape juice might be (1.050?) You want to raise it to 1.150. - A pound of sugar will raise the gravity of 1 quart (about 1 liter) by 1.160
If you go to http://web2.airmail.net/sgross/fermcalc/FermCalcJS.html you should be able to figure the sugar additions you will need. You will need the starting specific gravity (s.g.), the s.g. where you are at now, and what you want for an ending s.g. You probably won't get to 20% before the yeast peters out, but you might. I would suggest you step feed the sugar in, bring the s.g. up to 1.100 or so, ferment down to 1.050 or so and add some of the sugar to bring the s.g. back up to around 1.100, let it ferment down and add back up til you are done. I would also keep adding nutrients as you ferment at least thru the last feeding. Good luck with it, Arne.
Not trying to be harsh but sounds like you are looking for hooch not wine. A fortified wine (ABV above 18%) is best done by adding brandy or some other commercial distilled spirits to your wine. (Distilling itself is NOT discussed on this site as outside the scope of this site and few other reasons as well.) Some folks can coach you through step feeding your wine which may permit you to achieve your target ABV BUT again with most wine you will end it with a beverage that is pretty harsh on the palette.