On your Drone account, you can have up to 100 devices (or Drone serial numbers). To add a new device to your account, you'll just need to log in to your DroneFleet account and first select "Add New Vehicle" at the top of the webpage.
Each device has it's own serial number. So you'll need to first enter the serial number of the new Drone.
Then read the terms and conditions before clicking "I Agree".
Once that's finished, you should now see your new device on your dashboard.