Aamir,
The "Maximum License Usage" on this screen is telling you the maximum number of licensed users that you have had log into your system at the same time, it's not indicating the upper limit of how many users are are "allowed" to have.
You can update the "Licenses Purchased" on this screen (through "Edit in List") and then use this page to track your license usage to make sure you're staying within the licenses you purchased. Service Desk will not stop allowing people to log in if you exceed the "Licenses Purchased", and the "Maximum Licenses Usage" is only providing a snapshot of data from your environment.