On your CommBox Manage Dashboard https://commbox.glbth.com/index.html#/dashboard go to the top right corner and click on the user icon and select account to open your settings. Here you will find 'account credits' or alternatively you can reach the same page by clicking on this link https://commbox.glbth.com/index.html#/domainAccount.
Each credit is 1 day of a CommBox Manage Licence which can be divided among as many devices you like.