I think the correct answer should involve a calibration, some reference to what you interpret the value to mean. I wouldn't ask this question to a job candidat

I think the correct answer should involve a calibration, some reference to what you interpret the value to mean. I wouldn't ask this question to a job candidate, but in any context where it might come up, if the response is "10 because I am at least 90%tile on a speed or error rate compared to other candidates" then that's acceptable to me. It should make sense, though. If the response is "where 10 is having written a frequently referred to text on the entire subject I would score an 8," what does it mean.. Wrote 80% of a book? Wrote a book but it's referred to 4/5 as often as the best book? Neither of those are really practical interpretations. As usual, it's the way an answer is given that can make more difference than the response per se. If ever asked this kind of "rank on 10 your skill" question then giving a reasonable benchmark for your choice can improve the clarity of your communication, at the very least.