There's no simple answer to this question - it depends on a number of factors, including your age, weight, gender, and level of fitness. That said, 20 pull-ups is generally considered to be an impressive feat of strength.
If you're a man in your 20s or 30s who weighs around 175 pounds (80 kg), then 20 pull-ups is definitely impressive. If you're a woman of the same age and weight, then 12-15 pull-ups would be considered impressive.
Age and weight are important factors because they affect your body's natural ability to build muscle mass and perform exercises like pull-ups. For example, younger people tend to have more fast-twitch muscle fibers - which are better suited for explosive activities like pulling yourself up from a dead hang - than older adults. And people who weigh more often have an easier time doing pull-ups than those who weigh less because they have more body mass to help them generate the necessary force.
Finally, your level of fitness also plays a role in how impressive 20 pull-ups really is. If you've been regularly working out and doing other exercises that target your back and shoulder muscles - such as rows and chin-ups - then 20 pull-ups isn't as big of an accomplishment as it would be for someone who doesn't exercise regularly.