The number of seats you have is dependent upon your original license and how many additional seats you may have purchased. Your Monsoon Marketplace Server will be included when counting seats, so if you have 3 seats, this would enable you to have the server and two workstations running simultaneously.
If you want to switch to using a different workstation or simply have Monsoon Marketplace re-count your seats, you will need to turn off all your workstations, then restart your Monsoon Marketplace Server and open Monsoon Marketplace on that computer. Once Monsoon Marketplace is up and running on the server, turn on your workstations one at a time, opening Monsoon Marketplace on each one as you go. This will reset your seat count, allowing Monsoon Marketplace to recognize how many computers you are currently using.