I’d say that the appropriate number of tags for a post is “as many or few as seem to match the contents of the post”.
I expect that many posts that have just a few tags are that way simply because the tagger didn’t happen to think of the other possible ones. Or someone might be focusing on some particular tag and tagging all relevant posts with that, without also looking at those posts to see what other tags might be suitable.
I’d say that the appropriate number of tags for a post is “as many or few as seem to match the contents of the post”.
I expect that many posts that have just a few tags are that way simply because the tagger didn’t happen to think of the other possible ones. Or someone might be focusing on some particular tag and tagging all relevant posts with that, without also looking at those posts to see what other tags might be suitable.