Under one SecureTeen subscription, you can monitor maximum three devices simultaneously. These devices can be of any supported platform i.e. android, iPhone/iPad or Windows-PC. (You can check detailed installation instructions for each supported platform under 'Installation Guides & Tutorials' section of SecureTeen account).
If you want to monitor more than three devices at same time, you will need to purchase additional licenses, you can purchase multiple licenses under already registered email-ID. So that you may have all subscriptions/registered devices on same SecureTeen dashboard.
If you wish to purchase another license then please visit SecureTeen website at the URL below and use given 'Buy Now' option on this page: