According to our End User Agreement, by default you are allowed to register up to 5 devices on one account and use them all simultaneously. Users with paid subscription can delete one device per week.
If you want to add a new device, first you need to delete one of already registered devices. You can manage your account personally from your KeepSolid office at https://my.keepsolid.com/
Please go to Devices—here you can see the list of all your devices, and manage them manually.
Also, you can buy additional slots for your devices at Extensions, or right in the app through the Purchases tab.