The term "forum" can be ambiguous, so 1 license = 1 installation. Within that, you can create many "forums" (as in, things like our
announcements forum and the
pre-sales forum). If you try out the admin demo, you'll see what I mean in the "forums" section of the control panel. The admin demo shows what you'd get with one license (including the Media Gallery and Resource Manager add-ons).