This can be calculated like so :
(Max Size of message) * (Max frequency of message) * (Max Number of clients)
For example, let's say a message size is 1 Kb. With 1 message per second and 5000 clients, this will result in a required bandwidth of 5 megabytes/second (~40 megabits per second network speed). This is the maximum bandwidth needed for peak processing.
You needed to create a load scenario in your application to see the exact numbers.