I would go to this discussion.
https://www-secure.symantec.com/connect/forums/sd7-licenses
The current version of Service Desk MR1 is supposed to fill a license when a user access an advanced service desk page (ie. Advanced ticket page, KB article creation pages as well as many others). This is also supposed to be by IP address. All a single agent can have multiple pages open but only fill one license.
The trick is it doesn't work that way. If an agent opens multiple advanced ticket pages or other license filling pages in Service Desk it will fill that many licenses.
Also important to note is that MR2 is about to come out and according to the Release notes (which were only available for a few hours due to a mistake) that issue is supposed to be fixed.
I have been told by Symantec Technical support that MR2 is to be release "sometime this month."
We will see about that but if I were you I would wait for MR2 to be release before you install service desk.