Haha what, when I was drinking coffee daily it was like 12-14 oz a day, why on earth would anyone need 20 cups... D:
Some people do.
My mum probably has 20 cups across a day at times. But 20 is probably an upper bound. 10-20 cups isn't great if it means 20-40 teaspoons of sugar.
1 teaspoon of sugar is approx 4 grams of sugar. 20-40 tspn is 80-160g. It's recommended you don't have more than 30g of added sugar a day (so, sugar cubes, honey, etc), and only have 90g total sugar.
In fact, if 30 is the recommended max, that's roughly 7 teaspoons. So, for someone who has two teaspoons per cup, that's just 3.5 cups of coffee a day.