Each time you scan a particular target, that target will use one of your licenses for the next 30 days.

If a target is consuming a license and you delete it, the license will remain consumed until the end of the 30 day period. Should you decide you want to restore the target, just head to the targets page and click 'Licenses':

From there, click the green plus button:

Navigate back to your list of targets by pressing "All". The target has now been restored and will show as "Added. Ready to scan".