It’s actually counting everything that gets installed (even if hidden) in the library. That includes stacks, child-stacks, images, partials, externals, templates. Framework stacks usually include a couple dozen stacks. Joe tweeted the other day about the number of stacks in his upcoming version of Foundation – I think it was getting close to 100. Other stacks like Jack.stack include dozens of images and patterns that can be used in other stacks – those appear in the library too – so they get counted.
Part of the reason for adding that number was to show folks how much they’ve installed into RW. A lot of people don’t really keep track and are surprised when I tell them they have a couple thousand things in the library.
I figure giving people the running count every time they startup will let them see how each change they make affects the number.