What is the default number of concurrent requests that can be processed by IIS 7.5 on Windows Server 2008 R2?

And where would I find a setting to change it?

there is no max as it depends on your application settings, hardware, and .net framework version. By default under framework 3.51 and up the number of concurrent requests per CPU (logical) is 5000. (if not defined under 2.0 it's 12 but the reccomendation is to set it to 5000 like 3.5 and 4.0)

This is defined in aspnet.config:

<system.web>
        <applicationPool maxConcurrentRequestsPerCPU="12" maxConcurrentThreadsPerCPU="0" requestQueueLimit="5000"/>
</system.web>

This is the default and by no means the limit. The largest I have ever seen it set to is 999999 (i suspect they were afraid to set it any higher just in case). The best way to determine the right size for you is through stress testing the application.