Hmm... actually it's not listed in the rules .... it's in the "How to become a TamaGuide" topic (point #9) which asks "would be" Guides to demonstrate some humility - celebratory threads about posting prowess are not a good idea
I am happy that you are excited about making 300 posts, but I think it is not a good idea to encourage others to make threads each time they reach a significant number of posts... it will turn into a very spammy subject... and of course, everyone can see how many posts you have made on TamaTalk - it is recorded under your avatar along with the date you joined the site