If you mean a single category, it displays the number, if you want forum, add up the categories within it, on armorgames altogether add up all the sections.
Crazy talk firetail, you know how long that would take???
I think that is the only way to find out...but still, the sheer amount of time, you'd probably have to start an entire archive of what thread's you've counted, to keep track of what you've done so far, then keep going, probably take about, what? A month, maybe less if you work diligently...
Prepare yourself for awhile though in some categories like Forum Games, even if you're not counting the individual replies within a specific topic, the number of threads themselves is still quite astounding.
