There are actually 3 possible reasons for there being no number:
- There are 0 questions tagged with this tag
- There is 1 question tagged with this tag
- The tag is a synonym.
This is confusing and sometimes annoying.
I would suggest that it be changed to remove those tags that have 0 questions, and which are not synonyms, and place a number by the tags that have one? Like this (poor) mock-up:
This leaves a clear delineation between tags with questions, tags without questions and synonyms (mainly because the tags without questions no longer exist!).
I'm guessing that the ones with 0 questions attached might still be around because they haven't been "deleted" yet. If it's difficult/unwanted to remove these there could be a
x 1 by the tags that do have questions and either a
x 0 by tags with no questions or a method of identifying synonyms so people can differentiate between the three possibilities.