Insert the number (count) of subitems into generated documents

Suppose you're generating a document from a board item called "List of spare parts". This item has subitems such as Gearbox, Headlights, Tires, etc.

In this example, there are three subitems. You would like to display "3" in your document (or whatever the number of subitems happens to be.) Very easy: 

1. Add a Formula column for the subitems. The formula should be simply 1:
   
2. Click on the three dots next to the column name and click on "Show Summary on Parent Item"
   
3. This will create a new "summary" column on the parent item level. The default name will be "Subitems Formula", but you can rename it. In our example, the column should display "3", which is the number of subitems.
4. Use this summary column in your document template. For example, if you haven't renamed the default column names, your placeholder should be <<column_firstrow:Subitems Formula>>