How do you calculate Impressions per Month for widgets?

It is the number of times a widget is loaded on your front end. As an example, if you use the "Similar products" recommendations, which are usually on the product page, it will be the number of product views for the month. This is because each time the product is viewed, it will load the widget 1 time. 

If the widget has 8/12/24 product suggestions, these are NOT counted towards the impressions count.