That only matters if results differ between each person. If everyone has the same results, 18 people is plenty to prove that the results are the same for everyo

That only matters if results differ between each person. If everyone has the same results, 18 people is plenty to prove that the results are the same for everyone. It depends on the sample variance and the number of people. If you have high sample variance, you need a higher N to make it less likely that you have a sampling error. If you have very low sample variance, just a few people is enough.