You can limit concurrent connections from IIS 6.0 directly by following below steps:
1. Open IIS 6.0. (Click on Start -> Control panel -> Administrative Tools)
2. Double click on WebSites.
3. Right click on the website for which you wish to set the limit.
4. Click on Properties.
5. Select performance tab.
6. Select checkbox “Connections limit to:” under website connections.
7. Specify number of connections value you wish to set.
8. Click on Apply to save the changes.