The license entitles you to manage a certain number of devices, regardless of the number of servers. You can combine existing license keys at https://licensing.arubanetworks.com, and TAC can split license keys for you.
However, we only validate the performance of up to 5000 devices on a single server (see the document on choosing the right server hardware on the Home > Documentation page of your AirWave server), so you can't really manage 7500 devices on a single server.