An integer variable containing the number of trackbacks for this article.
It can also be used as a boolean variable; it is true if it is nonzero.
Example
In the example below, if there is at least one trackback for this article, the phrase "There are n trackbacks for this article." (where n is the number of trackbacks) is displayed. If there are no trackbacks, the phrase "Just wait, someone will link to this..." is displayed.
{{if num_trackbacks}}
There are {{num_trackbacks}} trackbacks for this article.