Yes, each camera must be on its own data plan. That being said, you can manage all of your cameras and pictures from the same account and app. In addition, multiple cameras of the same model and the same plan level (e.g. two Y2 cameras each with a paid Starter plan subscription) can share data with each other, so if one goes over its allotted data, it can borrow unused data from the other.
To activate your additional camera from the mobile app, simply click the ADD CAMERA button.
To activate your additional camera from your web browser, simply click the ACTIVATE NEW CAMERA button.