When Application (application Pool) is running in Integrated mode on IIS 7.0 or later you can change it. By default it is for 5000 for application hosted on .NET 4.0,

Leansentry Report

The application pool appears to be limited to X active requests being processed concurrently in ASP.NET by the MaxConcurrentRequestsPerCpu setting. We observed X+ active requests being processed by ASP.NET. Any requests above this limit will be queued
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You can try increasing the MaxConcurrentRequestsPerCpu setting on the server or for this specific application pool in order to allow processing more ASP.NET requests concurrently. However, if this limit is already high, increasing it is not likely to improve performance. Microsoft recommends setting MaxConcurrentRequestsPerCpu to 5000, which is the default for .NET 4.0 applications.

If You have application hosted on .NET 2.0 you could set set a DWORD registry value as follows

@ HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\2.0.50727.0\MaxConcurrentRequestsPerCPU = 5000.
Restart IIS.

For .NET 3.5, You Could set this in using aspnet.config.

aspnet.config file is a little known config file which is supported by ASP.NET 2.0 and greater. Generally it lives in the root of the framework folder, for example:

C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet.config
C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet.config
C:\Windows\Microsoft.NET\Framework64\v2.0.50727\aspnet.config
C:\Windows\Microsoft.NET\Framework\v2.0.50727\aspnet.config

Do check the aspnet.config file persmissions and remain it same.

In .net 4.0

In .net 4.0, it is set to 5000 by default. This attribute is not contained in the IIS configuration reference, maybe you could consult the asp.net or .net framework forum.

For more visit following links

https://docs.microsoft.com/en-us/dotnet/framework/configure-apps/file-schema/web/applicationpool-element-web-settings

https://forums.iis.net/t/1237694.aspx?Where+to+find+MaxConcurrentRequestsPerCPU
https://weblogs.asp.net/owscott/setting-an-aspnet-config-file-per-application-pool
https://docs.microsoft.com/en-us/biztalk/technical-guides/optimizing-iis-performance