They mainly want to know two things:

1. Is it increasing or decreasing? This requires them to ask you multiple times over the course of treatment or multiple visits. For these purposes, the first number you choose isn't that important. Just stay consistent and let that first decision anchor your later ones. But,
2. Can they diagnose you based on pain severity? They have a range in their heads that corresponds to "appendicitis", for example. If they tell you in advance whether you should be saying "8" for that, they'll bias the diagnosis; you just have to pick one. If they have a diagnosis in mind and the number you choose is wildly off, they may discard that possibility and look for a closer match for diagnosis. If you pick something slightly outside that range, they'll do more tests and exams and ask you more questions to see if the discrepancy is meaningless.

Bottom line, it's fine to choose whatever feels right in the moment. Probably stay away from 10's; a person with a 10 pain is probably unable to answer the question. You can answer "1" if they're examining the wrong body part; if you broke your wrist and they're prodding your leg, a 1 for your leg is totes fine, but make sure to give an answer that feels right when they poke your wrist.

(Besides, if pain severity is a factor in diagnosis, they can probably SEE you're in pain. People in a lot of pain flinch, and sweat, and stutter, and move gingerly, and protect the extremities that are in pain. They know that. They can see it.)