In deciding how many servers to run, the key issue for Shiny applications is less about RAM and more about compute. Compute requirements can vary widely by app. It’s best to try things out.
With Shiny Server Pro, you can figure out the footprint used (RAM and CPU) and the frequency of use. Its tools include the ability to constrain the concurrency of usage for any given Shiny app and the total number of processes started by default. As you learn how your Shiny apps are used, you can adjust your server configurations and add servers accordingly.