Forum owners make a lot of their membership numbers, this forum for example boasts over 9000 members, is that a real figure, or is it that historical it's no longer relevant to the forum today, and are prospective members influenced by it ?
Does not matter how many members you have on your forum, a few posts is still a few posts. People com for content and to discuss if they can't do one of the two people will leave.
It really doesn't matter how many members that you have. What really matters is how many are actively posting; creating threads and making posts.