IIS 8 on Windows Server 2012 doesn’t have any fixed concurrent request limit, apart from whatever limit would be reached when resources are maxed.

However, the client version of IIS 8, which is on Windows 8, does have a concurrent connection request limitation to limit high traffic production uses on a client edition of Windows.

Starting with IIS 7 (Windows Vista), the behavior changed from previous versions.  In previous client versions of IIS, excess requests would throw a 403.9 error message (Access Forbidden: Too many users are connected.).  Instead, Windows Vista, 7 and 8 queue excessive requests so that they will be handled gracefully, although there is a maximum number of requests that will be processed simultaneously.

Thomas Deml provided a concurrent request chart for Windows Vista many years ago, but I have been unable to find an equivalent chart for Windows 8 so I asked Wade Hilmo from the IIS team what the limits are.  Since this is controlled not by the IIS team itself but rather from the Windows licensing team, he asked around and found the authoritative answer, which I’ll provide below.

Windows 8 – IIS 8 Concurrent Requests Limit

Windows 7 – IIS 7.5 Concurrent Requests Limit

Windows Vista – IIS 7 Concurrent Requests Limit

Windows Server 2003, Windows Server 2008, Windows Server 2008 R2 and Windows Server 2012 allow an unlimited amount of simultaneously requests.