Here's a nice simple solution, the free ServerStats tool, that provides that info. It's from Mark Lynch (of Learnosity) and while a couple years old still works just fine. Besides printing out the total count of sessions and the count per application, ServerStats also shows some information about CF memory use, number of processors, and more.
It's totally safe to use (I've helped hundreds of customers use it in my troubleshooting consulting practice), and the CFML source is open for you to review.
Of course, some will want to point out that the CF 8 Server Monitor offers session and memory info, but for those not on CF8 Enterprise, this is a useful alternative. Also, some may want to point out that the solution described here uses internal objects that may be disabled (and are also undocumented and unsupported). I discuss both these points below. There are also some other approaches people take, which I also mention briefly.
Why care about number of sessions?
So why care so much about how many sessions are active on a CF server? If there are 10 or 20, or maybe a 100, does it really make a difference? No, not likely. But what if you find that there are 90,000? I'm not kidding. I was helping someone just last week when we discovered this. It's not as unusual as you may think.
Often when I'm helping people solve problems with their CF servers, one of the first things I want to help them discover is just how many sessions they have active at any time. With the potential impact of spiders and bots creating huge numbers of sessions, which could have an impact on memory and also on other aspects of performance, it's definitely one of the first things to look into.
So how's Mark's code tracking them?
Mark's blog entry (pointed to above) is a summary of things he'd discussed in a few blog entries about how to leverage internal CF/Java objects to provide the info, and that one entry offers ServerStats as a downloadable zip with a single CFM file and a directory of CFCs and custom tags. You can just extract the zip into a web-accessible directory and run it. No need to create custom tag mappings, etc.
Many folks have known about and blogged about these internal CF/Java objects, coldfusion.runtime.SessionTracker and coldfusion.runtime.ApplicationScopeTracker, over the years. They're undocumented and unsupported, so you use them at your own risk, but again I've confirmed that Mark's code works against CF 7 and 8 (and I'll assume 6 as well.) You can google to learn more from him and others on these.
CF8 Enterprise has other solutions
Of course, those on CF 8 Enterprise (or Developer) have still other approaches to this, whether in the CF8 Server Monitor (see its active sessions page) or the Admin API's servermonitoring.cfc. But if you're not CF8 Enterprise (and even if you are) this solution can help.
Update: FusionReactor Extensions for CF
When I first wrote this entry in 2009, FusionReactor did not offer any session tracking, but does since the introduction in FR4 of FREC (the free FusionReactor Extensions for CF), which added tracking of CF sessions in its realtimestats.log. For more on that, see my blog entry, Tracking #ColdFusion sessions within FusionReactor, by way of FREC logging.
Finally, note that in FusionReactor 5, the FREC functionality is built-in, and besides the logging of session count in realtimestats.log, there is now a chart of sessions offered in the Metrics>Custom Series option on the left. Choose the option for ActiveSessionCount in the drop-down on the top right, if it's not selected by default.
(For any who may wonder, SeeFusion does not offer session tracking info.)
JRun Metrics and other solutions
And yes, there's still another way to get at least a count of all J2EE sessions, using the available JRun Metrics. While it has the benefit of tracking the info over time, it does track the session count only if you've enabled J2EE Sessions in the CF Admin, and even then it tracks only the total of all sessions, not a count per app.
Of course, there have been various solutions offered to solve the problem of session tracking, such as those that track sessions in code (within your application) by storing them in an array in the application or server scope, and so on. The nice thing about using the internal CF/Java objects is that the apps being tracked don't need to be altered at all.
Potential Security Gotcha
Of course, therein lies a potential security risk. Anyone running such code on any CF server can have access to the sort of info that these internal CF/Java objects offer. The example above (Mark Lynch's) doesn't really expose much to worry about (unless the names of apps on a server is sensitive, where one user on the server shouldn't know that another app exists on it, which may be true in a shared hosting environment.) But more important, if one digs deeper into these objects, they do expose more details, including the values of variables in session scopes across all applications. That could be considered very sensitive.
As such, there have long been a couple of solutions that Adobe has provided to enable an administrator to shut down the use of these internal objects.
First, in CF 5 and above, it's been possible to set security for the entire server (or a given app in CF Enterprise) to make it impossible to call any Java objects (yes, you could call Java objects starting in CF 4.51). To learn more about this option, called Resource Security in CF Standard, and Sandbox Security in CF Enterprise, see my 2-part article series for the Adobe Security Developer Center from Sept 2002, "ColdFusion Security, Part One: Understanding Sandbox/Resource Security" and "Part Two: Sandbox/Resource Basics", both available at my articles site.
That approach is a little brute-force though, and throws the baby out with the bathwater if you have legitimate use of java objects.
So in CF8 finally, Adobe added a new feature that permits an Administrator to disable JUST access to the these underlying internal CF/java objects. See the CF Admin "Server Settings" section, "Settings" page, and its "Disable access to internal ColdFusion Java components" checkbox. The option takes effect immediately.
It's certainly more effective if one wants to control to this info, but of course it will make code that relies on it fail, with an error like:
Permission denied for creating Java object: coldfusion.runtime.SessionTracker
I'll point out that since most of the folks I help with run their own servers, they're just not worried about disabling these internal java objects, since they can control what code runs on the servers.
So if you want to quickly find out how many sessions you're running, per application, check out Mark's code. You can also get much more info from these internal objects, if you want to explore. I'm working on a tool to provide some still more powerful information that I started working on years ago based on these objects. But until that's ready, I had occasion today to point out this working alternative to a customer, and thought I'd pass it along here.