I'll detail the limits per version/edition below.
I'll also offer a (possibly surprising) workaround that can allow you to get even more requests through IIS, even for a single web site.
(Before I elaborate on that, note that there is a separate issue if you're finding that CF doesn't let you see more than 25 requests at once. That's instead due to a setting in CF/JRun, the maxworkerthreads setting. For more on that, see this blog entry.)
That said, this is a problem which could affect anyone regardless of the app server they may be running behind IIS. (And yes, I do realize that for some, the answer to this problem will be, "see, that's another reason to run Apache." We get that. Let's just focus on this problem for those who choose/have to remain on IIS.)
I was doing some load testing noticed that I couldn't get beyond 3 requests running at a time. I suspected the problem may be IIS.
So at first I tried just making the same requests against CF's built-in web server (such as might be running at port 8500, which can be enabled/configured during CF installation or manually).
Sure enough, CF could execute more requests at once (up to the "simultaneous requests" setting) through the built-in web server.
So that told me the problem had to be in IIS. I did some digging and found the documentation that affirmed what I was seeing.
(Some may recall there used to be a limit of 10 for IIS on Windows XP, so it may surprise some to run into an even lower limit on later Windows versions.)
IIS 7 does have a "request execution limit", for all but Server editions of Windows
The first resource I found was with respect to the limits in Vista. This page says specifically: "With Starter and Home Basic editions of Windows Vista, the simultaneous request execution limit for IIS is three for Windows Vista Home Premium. ...For Windows Vista Business, Enterprise, and Ultimate edition...the simultaneous request execution limit is ten. ..With server editions of Window, IIS 7.0 has no request execution limit."
But I'm sure some reading this will be using Windows 2008. I found a page discussing its limits: for the Basic & Starter editions: 3 requests, for Premium: 3, for Pro: 10, and for Server: unlimited.
I happened to be running Windows 7 when I hit the limit, and while I've not found a document stating the limits specifically for Windows 7, I can again confirm the same limit of 3 on my copy of Windows 7 Home Premium, so I'm sure things are the same, to whatever degree the edition names parallel the other versions. (It's curious that the learn.iis.net page above does discuss both Vista and Windows 2008, but makes no mention of Windows 7.)
A workaround for this limit: multiple worker processes
With respect to this limit, here's a valuable bonus tip that I've not seen documented anywhere: the limit is really per application pool, or more technically per worker process. So you can certainly get more request simultaneously against your box by using multiple app pools.
But perhaps you want to have more requests against a single site, which obviously can be connected only to a single app pool. There's still good news: you can increase the number of worker processes per app pool in the "advanced settings" for a given app pool (right-click on the app pool), increasing "Maximum Worker Processes" from the default of 1. (Some will recognize that as being the same as what was known as "web gardens" for app pools in IIS 6.)
For those new to them, whether creating new app pools or more worker processes for an app pool, for each new worker process, you'll see a new w3wp.exe in task manager.
A caution, for those using ASP.NET, about setting more than one worker process per app pool
Finally, there's a caution to consider if you decide to increase the number of worker processes. At least back in IIS 6, I documented that if you're running an ASP.NET app using sessions that are "inproc" (or in memory), the default, there is a problem with using multiple worker processes (web gardens), in that the sessions are not replicated among the worker processes. That may not be an issue for the OP, so I'll say you can learn more at an entry I did a few years ago.
Did any of this help you? Let me know below.
Update: Windows 8 and IIS 8What about Windows 8 and IIS 8? See this info. This is referring to the client version of Windows 8. The server version is called Server 2012, and like the server edition of 2008, there are no request limits (according to comments on that blog entry).