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By default IIS on IIS is limited to 10 connections. This can be increased to 40 connections. To do this, find the adsutil.vbs script (should be in c:\inetpub\AdminScripts or similar) and run the following command:

adsutil.vbs set w3svc/MaxConnections 40

To execute from a command prompt or from Start > Run execute the following command. Update the path to adsutil.vbs if needed. %SystemDrive% is C: by default unless you have installed Windows on another drive.
%SystemDrive%\inetpub\AdminScripts\adsutil.vbs set w3svc/MaxConnections 40

You should see this output after executing:
MaxConnections : (INTEGER) 40

Important: Do not increase the limit to more than 40. This is the hardcoded limit and will cause problems if you set it higher.


If you have increased the limit and you are starting to get the 'too many users' error you can do the following:

- Run the IIS Administator ( Start > Control Panel > Administrative Tools > Internet Information Services )
- Right click on your website and select properties.
- Under 'Connections' try reducing the connection timeout. The lower the value the faster the open connections will be released.
- You can also disable HTTP Keep Alives as a final option but this will reduce performance since the browser has to create a new connection with every page viewed.
- If you have done this and still get errors you need to upgrade to Windows Server.


Source:
http://weblogs.asp.net/cazzu/archive/2003/10/10/31476.aspx