yes you can add more numbers of user but at a time only no license user work.
10 license you have then only 10 user work at a time
but license installed on any no of users.
you can also add new license in existing license using purchase new licenses.
In this case license available first come first serves basis.
When one of user release license its available for others.
In this case please use type of licenses level
basic user have only basic license
Advance user have advance license level.
Please check below links,
The audit log captures license borrow, return, and enterprise single use authorization activity. You can view the log file using the ArcGIS License Server Administrator.