Yes, for the thing it is good at.

Five participants will give you a strong view of the core usability problems in a flow. Jakob Nielsen’s work is the usual citation, and our own experience across hundreds of sessions a year matches it. The same three or four serious problems turn up again and again, and by the fifth session you are mostly confirming what you already saw.

Where five is not enough is when your customers split into genuinely different segments with different needs. If that is the case we will usually recommend ten, and we would rather work that out with you in the kickoff workshop than discover it halfway through fieldwork.

What five people cannot tell you is how many of your customers have the problem. That is a quantitative question. Qualitative research tells you what is broken and why. Your analytics tell you how often.

The methodology is what validates the data, not the headcount. Five people recruited badly will tell you less than nothing.