Anecdotal, but I've observed quite a few in-person usability studies. After the the third or fourth person you've usually discovered all your're going to with y

Anecdotal, but I've observed quite a few in-person usability studies. After the the third or fourth person you've usually discovered all your're going to with your current test script and stimuli. You probably could design a study that would continue to uncover new issues after many tens of people, but the test would probably be two hours long and would be unreliable based on that fact alone. The NN Group article linked by the sibling comment holds true in my experience.