Your Maps Made Easy subscription comes with a fixed allotment of iOS devices that are allowed to use the account at any time. It is possible to reset the list of devices that are authorized to access the account one time in any six month period.
In your account Profile you will see a button labeled 'Reset Device List'. This will reset your authorized device list and your subscription will again have the full number of devices available.
If your subscription is newer than 6 months or you have reset your device list recently the button will not be present and you will be shown the date at which the next reset is available.