Here’s an element of my teaching philosophy.
If one student has a question, maybe just they didn’t understand the topic.
If several students ask the same question, it probably means I didn’t explain it well enough.
If I get the same question from 3+ students, I email the answer to everyone.
Yeah, 3's my threshold too (physics prof, here). Unless it's an actual error on my part of course, then I send something out to everyone as soon as one person points it out (or I catch it myself).
I had a prof that would grade all tests a question at a time. That way she would notice how many missed a particular question. If a certain percentage missed it (I can’t remember the exact % anymore) she would remove the question with the thinking she didn’t cover it well enough.