Each account comes with the ability to upload and keep 30 Prevues in your library at any one time. This limit only refers to Prevues - you can keep an infinite amount of annotations, notes and projects.
Once you have reached 100% of your limit, you will be unable to upload additional Prevues until you delete one or more existing Prevue.
You can view how much space you have remaining at any time by visiting your Account section. This will also outline ways you can earn extra space (find out more)
The above shows the message that will appear in your Dashboard when you run out of space. You will notice that the 'available space' bar in Your Account will change colour depending on how close you are to your limit.