For qualitative research, aim for under 15 participants. For quantitative work, run a power analysis with GPower software to find your minimum sample size. The real answer: whatever your review committee will accept.
How many participants or data points are actually enough for a defensible study?
The really simple, crass answer is what your committee will accept. Ultimately, it depends. First of all, is it a qualitative or a quantitative study? Qualitative, typically under 20, hopefully under 15. Quantitative, whoa, I can't answer that one easily. The answer to that one is do the G*Power test and look up G*Power, and that will tell you the minimum number needed to be able to answer the question questions you're asking.
