In order to add a device, you must install the spy app on a target device and do an authentication with your username/password on the app.
According to the paid package, you can have from 1 or 3 or even 50 devices you can add to your account.
Every time you do an authentication with the app on the target device, that means you attach/add/link that device to your account. You do 3 times that mean you added 3 devices.
In order to remove the device, you must log in to the user control panel website, click Device Setting menu -> Remove Device and then you can confirm to remove the device from your account.