There is no limit on the number of applications per account. Admins of an account have access to all applications belonging to that account. Other users can be granted explicit access to applications via the admin panel.
Each application is required to have a unique identifier. The identifier must be unique amongst all applications in RESTful Metrics. If your application is a web-based application, you may want to use the application's domain name. The identifier is not public and does not appear anywhere on dashboard but it's important you use the same identifier (case-sensitive) consistently to ensure all of your emitted data points are properly associated with the proper application.
Before you can begin collecting data points for an application, you must create the application under from the accounts page on the dashboard. Once your application has been created, you can begin using the unique identifier in your application when emitting data points.