In addition to the designation of Junior or Senior Member, you'll also notice there are white flowers directly below the member's name in their posts. You start with zero flowers, and as your post count increases, the number of flowers will increase. If the first flower is purple, it means that member has made a donation and is considered a Forum Supporter.
I can't tell you what number of posts are required to change your status from Junior to Senior or to give you another flower. I do kind of remember that the first flower came pretty quickly (maybe 30 posts?), but then the number of posts to get additional flowers increases by quite a bit. As you can see, I currently have three. I'm thinking I got my third one at 500 posts, but I don't think a fourth one will come until I have 1,000 posts. Not sure, though. And not that it matters. It's just something interesting to talk about.