Refer to this post, above, for some tips on setting up front|back matter for compile groups, so you aren’t having to remember to shuffle those around.
The <$wc100> placeholder itself (and other rounding variants), from the title page indeed only counts what you’re compiling at the moment. To do otherwise it would have to waste a bunch of time compiling everything outside of what you selected to compile, only to count it and then discard it.
The default settings for Project Targets is to only count from the current compile group you have selected, so that will work as you want. The Project ▸ Statistics... panel on the other hand is set up to count everything by default. Tune it in its options tab, at the very top.
A thread that links to this one may also be good reading (it goes over section types a bit more), and as I note above, these two threads are just a start, there is more that may be worth searching for. Think of how people might ask about it, like using the word “trilogy”.