Next to the comment counter, I can't seem to figure out what the bracketed number indicates. For a second I thought it meant new comments, but I've seen some posts that have 2 (+2) and when I open it, it still only has 2 comments.
It's how many new comments there are since you last opened the post. If it says 12 (+2) it means there are 12 comments but last time you opened it there were 10.
It would only show on posts you have previously looked at. So in the 2 (+2) scenario, you previously saw the post when there were no comments, now there are two.