You can have as many trackers as you want on your account, but different service levels limit the number of drivers you can create, all service levels come with at least 1 driver per tracker.
To add an additional tracker to your account, simply log in to the website then follow the activation instructions - when you visit the activation page you will be asked to enter the account password and the activation code for the new tracker, after activation, you will see a new vehicle on your account, ou can rename this vehicle on the Vehicle Settings tab.