How many users can Team System Web Access support?
A: Microsoft posted a white paper on this.
From the whitepaper:
"Test results indicate that Web Access starts to hit the upper limits once the total number of work item objects in the memory is around 180. This translates to approximately 100 concurrent users with typical usage patterns."
"Test results indicate that Web Access can support up to approximately 500 users." (working with a minimum amount of data)
Download: Team System Web Access 2008 Scalability Limits White Paper