I finally found the answer to this common question.
There is wonderful post at http://blogs.msdn.com/dstfs/archive/2009/07/02/determining-team-system-web-access-usage-levels.aspx
in which JimSau references a scalability white paper http://www.microsoft.com/downloads/details.aspx?FamilyID=ee378d7b-45c3-4690-bd06-a060cfa43b06&displaylang=en
To sum it up: Keep it at less than 100 concurrent TSWA users. Jim also provides a report to show how many users are accessing the system.