I put in a ticket with Aruba TAC. The engineer provided the following response and supporting attachment:
Say if you have purchased a Airwave 500 device license you will be able to add 500 devices to that server. To know the remaining number of devices that can be added to your server you have to subtract the number of devices (up+down) that your server currently has with the actual license count (ex:500).
In the above screenshots the license has support for 500 devices. So we need to add the number of up and down devices then need to subtract them with the license count. Ex: 500-173=327. So with respect to the above license we can add 327 more devices to the server. Like that you will be able ot calculate for your server. Please let me know if that helps.