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The number of comments are not available directly in the theme on its own.
To do so, you will need to change the code to include
$xoopsTpl->assign('commentnum', $story->comments());
somewhere in the php file, fetching the data from the database - provided that the story object is called "$story".
After that, you will be able to place a <{$commentnum}> anywhere in the related template to display number of comments.