It may be the positioning of the slashes, but I'm having trouble understanding what you're asking. How quickly what posts arrive? Forum posts? I'm not sure about the "how many have to be" part.
If you're talking about how the bulletin board software denotes things with the little icons, I'm not entirely sure. I think it's got more to do with frequency, but that requires a certain amount of quantity, too.
It's been ages since I've done back-end stuff with phpbb (been working with vB, mostly), but if I remember right, it's the number of replies that flags something as "popular". It might, however, also factor in number of views, but I don't believe so.
