GRID Virtual PC license usage

I have a question about how the license usage works under VMware horizon. We have multiple desktop pools bases on instant cloning.
I have in totaal 500 license for GRID-Virtual PC and 500 concurrent users for VMware horizon.
Is someone login the desktop does it cost one license. Or every desktop that i have available cost a license.

Also when does it gives a license back. It looks now i always have to wait 1 day and not when i delete the machine