My way to achieve that would be a web service or an app service. Two possible scenarios; 1. update on each new account 2. retrieve values when viewing
First one; Add to the track application with web service, which update the user # value per application. Call this web service when a new user is added. Since adding new users is different then editing a user, this can be done easily
Second one: Add a web service to the business application, which returns the current # of users. Call this web service each time the track application needs these values.
Choice between these 2 depends on number of applications, amount of user changes and how much the manager is viewing these values. Add the integration at the most costly moment. (which is done the least)
Mendix has an API that is build in by default for this. Here is the API documentation You could use the get all users call to monitor which users are being created in each app. The API keys is something you can activate from the sprintr tool.
Yes, take a look at App services