As a general rule, a typical quad core processor such as Intel Core i7 or Xeon E5 series can handle over 1000 simultaneous, or concurrent, users running an Iron Speed Designer application of average complexity. This limit is not imposed by the size of the application but rather by CPU ability to handle concurrent requests.
Improving Database Performance
Improving Application Performance
View State Storage Options for Improving Performance
Windows 2003 Application Performance Suggestions
Running Applications on Multiple Servers