Hello Outrageous,yes the license gets released once the count of machines are less.
You can use that license for other machines.
when client offline, the entry is still in SEPM, the license is counted
when client is deleted from SEPM ( default 60 days ) license is released
https://www-secure.symantec.com/connect/forums/reusing-sep-license
Here is the symantec document
The following procedure will cause the SEPM to remove clients from its database that have not checked in within the past 24 hours, and thereby reclaim these license seats
http://www.symantec.com/business/support/index?page=content&id=TECH168121