The easiest thing, I guess, is for you to unilaterally decide that the millionth post (prize winner) will be the post that has the post ID =1 million. And then look for that. In other words your millionth post count will include posts no longer present because of deletions.
Failing that, as you approach your milestone, calculate the difference (= number of deleted posts); keep it up to date with any new deletions, and look for post ID {1 million + difference}.
The latter will be harder for a would-be prizewinner user to predict with any accuracy.