You can backup as many computers on your account as you wish, all you need to do is install the application on another computer and login to it with your same email address and password.
You can then either transfer an existing backup license, if you no longer intend to backup the device it is currently assigned to, or you can purchase a new license for 29.95.
Additional licenses can be purchased by logging into your Control Panel on the web app and clicking the My Devices link on the left sidebar.
Here all your devices will be displayed in a table, any without backup licenses will display a button labelled Add+ in the license column.
Clicking this button will provide options to transfer a license from a previously licensed device or purchase a brand new one. Additional licenses are 29.95 per year. You can, however, use the sync folder on computers without licenses. You can also restore backed up files from other devices without a license on the restoring device. Mobile devices don't require licenses at all and can be backed up license free.