The number of concurrent users of an application on shinyapps.io is limited only by the number of application instances, workers, and connections you have configured. The configuration options are described in detail in our Scaling and Tuning article.
The default configuration for an application allows 150 simultaneous connections across three workers on a single application instance. Depending on the nature of your application, this may not be the optimal configuration for you, so we recommend reading the above article to familiarize yourself with the performance tuning options.