After purchasing AirServer, you will receive a license confirmation email, with your license key. In that email, you should have received a link to manage your activations (on older emails this can be called "Your Management Interface"). Visit that link and you will be able to see a list of current activated devices and previously activated ones (grayed out). You can also deactivate any unused devices.
In case you are unable to find the confirmation email, you can use our license lookup tool to resend the management link to the email address that you used while purchasing AirServer.