IIS Concurrent Request Limit depends on your web server's Hardware.
By default, request queue limit is enabled in IIS 6.0, and the default value is 1,000 requests. When resetting the default value, remember that a value that is too high can prevent other applications from having an equal share of the server resources and a value that is too low can cause the server to return 503 errors, or a disruption of service for the clients.
How To configure a request queue limit in IIS
1.
In IIS Manager, expand the local computer, right-click the application pool you want to configure, and then click Properties.
2.
On the Performance tab, select the Limit the kernel request queue (number of requests) check box.
3.
In the text box to the right of Limit the kernel request queue (number of requests), type the maximum number of requests to allow in the request queue, and then click OK.