I was recently asked this in an interview but was not sure how to answer. I tried saying that it would depend on time, the readiness of the samples, and the cost of the tests but the interviewer said to ignore those factors. I said there should be enough samples to make have a statistically significant outcome. So I guessed 100 samples. The interviewer seemed happy with the answer but wanted to know the minimum sample size. So I took another guess at 10 samples and the interviewer did not like that answer.

What is the correct way to answer this question??