Crikey! That's extremely strong :o
The normal advice is to add half a teaspoon of potassium sorbate per gallon/5 litres.
The stronger the wine, the less potassium sorbate you need, since the alcohol itself will inhibit yeast reproduction. But it won't do any harm.
Back sweetening is adding sugar (or other sweeteners) after fermentation is complete. That's why you need to inhibit the yeast, otherwise it'll start fermenting again. You might want to add sugar to achieve a "demi-sec" or even a sweet wine. Not everyone likes their wine to be too bone dry.
If you've made something in the 16-20% range, I'd expect to need to add some sweetness, and maybe drink it more like a vermouth than a wine.