Monthly tracked visitors are calculated by combining unique anonymous visits as well as unique identified visits in a given month.
To add to this, if an anonymous visitor arrives to your page twice they will be tracked as one monthly unique visitor in the given month regardless of how many events or visits they have. The same is true for identified visitors as well.
The only adjustment to the rule above would be if the visitor arrived from a different device, this would prevent their anonymous or unique_id from being captured and would result in a new MTU.
If you have any questions about this please feel free to start a conversation with our support team by writing to firstname.lastname@example.org or clicking the chat icon to the top right of your screen.