Great questions. (Bias: I'm one of the developers behind Shiny Server.)
Regarding your second question, in the open source Shiny Server, one R process is created for every Shiny application you deploy, regardless of the number of concurrent users. We now offer Shiny Server Professional which offers, among other things, the ability to support an application with multiple R processes and configure exactly how you want this to scale.
Regarding your first question, Shiny Server certainly can do these things, but you'll find that the limitation is in your Shiny apps. We've seen the simplest apps seamlessly support hundreds of concurrent users on a single Shiny process (doable in the open source version). Most apps of substance, however, would require more processes to create a seamless experience once you get into hundreds or thousands of concurrent users. With Shiny Server Pro, you'll be able to scale that as far as your server's resources allow you to.