Yes, the current limit is 5 active devices per account. Deactivated devices remain listed on the 'My Devices' tab, but do not count toward the 5 device limit. Follow the steps below to add an additional device to an existing account.
Install Mobile Defense on your additional device
Upon opening it, tap Sign In
Enter your existing login credentials
Tap Sign In
Once you've logged in successfully, go to settings and give your device a nickname so you can tell them apart in the web portal more easily.