How do I know how many updates I have left?

The Sc App interface shows the number of product updates waiting to be validated and sent to you Shopify backoffice on the menu 'Pending updates':

You can also see how many updates there are left within your subscription by clicking on the 'Pending updates' menu:

You will also find this information when you open the 'Elements to update' window, accessible by clicking on 'See all pending updates':