As you can see here, there are (as of January 2020) 5,931 questions tagged elementary-number-theory and number-theory. (For the record, there are respectively 28,327 and 31,874 questions in these tags, so that's not insignificant).
It's always been my impression that these sort of tags are mutually exclusive: set-theory vs elementary-set-theory, abstract-algebra vs algebra-precalculus, probability vs probability-theory, etc. In any case it makes zero sense for a question to have both tags, it's just redundant; they can always be tagged just elementary-number-theory.
I believe the differentiation between abstract-algebra and algebra-precalculus works because they are names of actual classes in standard curriculums in the US, and there are very active users (user?) dealing with set theory questions. However, things didn't work out so well for probability theory, as we can see in the question I linked at the top, and number theory is going the same way.
Rather than dealing with specific cases, I'd like to ask general questions:
- Is differentiating tags by level useful?
- Is differentiating tags by level feasible? How?
- How to decide when a tag needs to be split like this? How to decide what the threshold should?