The problem is that it isn't necessarily dependant upon how popular a forum is, but rather based upon how active your members are. If your topic is of a nature where most members are very active and constantly engaging in thought-provoking discussion and conversations, then that is different from a support site where most people only post questions when they have a problem with something.

When you're starting out, I would start with lower numbers, because that encourages members to continue posting to the site, and is a good way to get a new forum off the ground.

Quality over quantity is definitely the way to go. Good luck.