Not at all. To be statistically significant, you typically need 1000 or more test cases / individuals.
It depends on what you measured. For example, if you ask 20 people a yes/no question and they all say yes, the chance that they would all agree by chance is 1 in 2^19.
If they are selected randomly
(On the other hand, if you go to a board game convention and ask 20 people if they like board games...)
Sure, but that has little to do with sample size.
It’s very difficult to randomly sample 20 people. And impossible if anyone is or even could be dropped from the study.
Do you include prisoners, homeless, those with diminished mental capacity, or even people on the run from the law etc.
That isn’t how statistical significance works at all: