What is a merge? Great question! Merges are key concept for Formstack Documents.
In the broadest sense, the combining of a set of data and your document template constitutes a merge. Each unique delivery will count as a separate "merge" towards your monthly limit. For example, if you have an email delivery and a Google Drive delivery for your document, generating that document will count as 2 merges.
If you're using a Data Route, each document merged in the Route will count toward your monthly merge limit. For example, if your Data Route contains 5 separate document templates, each of which are set to be included in that route every time, each time that Data Route "fires", that would count as 5 merges.
You can use some tricks, like adding a CC address and using conditional deliveries, to help reduce the number of documents delivered, and therefore the number of merges.