Do you mean actual posts or discussions? If you mean discussions, this discussion is number 1,846,199. (Seriously; the number is in the url.)
If you take an average of 10 posts per discussion, that means over 18 million posts.
may be. There could be more than 20 millions posts. Who knows I have 357 number attached with me. There are more than 100,000 users and if multiplied with 300 then it would add up to 30 million posts.
Only admin would know what is the truth.