You should review the Performance Section of the SignalR Wiki on GitHub. This provides some really good guidance for the following:
- IIS Configuration – Max Concurrent Requests Per Application
- ASP.NET Configuration – Max Concurrent Requests & Request Queue Limits
Additionally, it states the key Peformance Counters to monitor when performing load testing of your application. Which IMO you should do to truly have an understanding of how your application is going to perform and make adjustments as necessary for your specific scenario.
Finally, there is a link to Crank the recommended tool for generating client load with SignalR.
Update: Link to Crank is now current.