Assuming you are trying to stay on a single screen (i.e. not an approach in which only one entity instance is added per screen), then you could modify the velocity template. Add a check to verify you are on the screen which is collecting the desired entities (i.e. screen name/title or custom field on screen) Depending on the exact behavior you are after, you could disable/hide the submit button until at least one instance has been added and also disable the add instance button once the maximum number has been reached. Another option is to leave all buttons as is and pick off the submit action, perform the check on number of instances and render a message (in a pop-up?) indicating the problem - i.e. prevent processing the submit action until the checks are satisfied.
After building a project, you can find the velocity templates in release\web-determinations\WEB-INF\classes\templates
Oracle Web Determinations Template Reference: Oracle Web Determinations Template Reference Guide
The velocity developers guide is available at: http://docs.oracle.com/html/E38272_01/Content/Technical%20Reference/Velocity_Templates_DG.htm