You add it in exactly the same way as the word "words" was added.
The last line of the template is concatenating strings together into a longer one. Changing this to the following should work:
Code:
subtitle = strcat(field('#pages'), ' pages', ' |', word_count, ' words')
You can combine the second and third parameters, but, the above is what I would usually do to keep each part of it separate and easy to change later. Though it probably has a small performance hit.