I have a mod that puts a goal and progress bar on the forum index:
https://vborg.vbsupport.ru/showthread.php?t=255898
But there's no way to know for sure that is the 2 millionth post because as was explained posts are constantly getting deleted.
If you edit your postbit (or postbit_legacy) template, find: $post[message]
Right above or below it put:
Code:
<if condition="$post[postid] == 4451920">
This is the 2 Millionth Post!! <br />
</if>
*That's the number calculated by zachery, adjust it to your needs.
This will put a note in your 2 millionth post so someone will know as soon as they make it. Tell them to contact you if they see the message.