Within Teamwork Desk, as an admin, you can review monthly billing details.
To do this click on your profile icon to the top right of Desk and choose 'settings':
Choose 'Billing' from the settings area:
Select usage from the left pane to view the usage:
If you add extra agents during your subscription period, the difference in subscription cost based on this change will be calculated pro-rata in your next invoice.
Note: You will not be charged during your free trial.
For more information see: How billing works