If you select the landing page template in design studio, you can see the template ID in the URL.
For example, in my instance I see "#LT1B2ZN11" in the URL for the standard template. The template ID is the number immediately after "LT" and before the "B", so it's 1 in this case
I've never used the Template ID constraint, so I'm not 100% sure that it is always populated.