How is the number of active users / licenses used calculated?

The list of active users / used licenses can be counted by using the Users tab on DESelect Admin Panel.

To access Users on Admin Panel:

1. Navigate to the Admin Panel.
   
2. On the Admin Panel sidebar, under Administration, choose Users.

On the Users page, you will have the list of all the active users indicated by the Active flag. 

In case a user is active in multiple business units, it still counts as one user.

Note that removing access for a user in Marketing Cloud to the DESelect Installed Package does not automatically change the active flag on a user. 

Find out here how you can transfer user licenses.