They only gain posts if they are mentioned, referred to, etc. For an example...um. If you feel like reading a little, try the
roleplay Shrewd and I have going in Roraldi. At the beginning of the second page, one of her characters leaves-but not really. Although said character is still in Roraldi, he doesn't come up in any of her posts, so does not gain any posts towards his count.
Make sense?