Well, technically, the post number would be correct. Just because a post was deleted doesn't mean that it wasn't made.
If you're looking for the 1,000,000th post that is currently visible on your board then you're not going to be able to do that without running a SQL query. You'd want to try something like the following:
Code:
SELECT `postid` FROM `post` WHERE `visible` = 1 LIMIT 999999, 1