Here are the steps below to find the number from the desktop app, and to mass create the preplans.

Go to properties-->New Pre-Plan

In the filter Check "property Never Inspected" then search

The bottom corner of the screen is the number of properties without preplans. Also, you can mass create from here. Just click the "Select" box (ensure the check is in there for select all), Assign a due date, and then create.