Hi Chris,
Exclaimer is installed on to either the gateway server for external disclaimers, the mailbox servers for internal disclaimers and both servers for both internal and external disclaimers.
In the scenario you have given I would assume you require external disclaimers only and therefore would recommend you install onto the server in the main office in the US.
The license applies for the total amount of users mail being disclaimed by Exclaimer.
If you do have any further questions, please let me know. I will be happy to help.
Best Regards
Joshua Clarke
Exclaimer Support