What does tag(s) mean?

While I was adding gpg to my favourite tags, I saw there was no number mentioned besides the tag. If I search and add other tags, they present numbers like key-rotation(10) which means 10 questions with this tag, but what does gpg(s) mean ?

The s is short for synonym. It is shown whenever a tag is synonymous to another tag.
Since we handle GPG as a synonym of PGP, the system shows that (s) instead of a question count. That's because technically, all GPG related questions are sorted under the PGP tag — which does show the number of related questions.