While ux researchers are generally less concerned with sample size than quantitative researchers, they should be mindful of strength of evidence.
As Philip Hodgson outlines, a researcher can collect good data and bad data. An individual's opinion about a design is bad data; observing an individual struggle with a design as part of a contextual data is good data. Generally speaking, the closer we are to observing the user engaged in their natural behaviour, the stronger the evidence.
Hodgson says that a researcher should strive to collect data that is valid, meaning that it actually measures what we are trying to measure, and reliable, meaning that another researcher performing the same study would come to the same result.
Hodgson, Philip. “User Experience Research and Strength of Evidence.” UserFocus (blog). Accessed October 10, 2020. https://www.userfocus.co.uk/articles/strength-of-evidence.html.