This is post 999440. According to the information here at the time of posting:-
https://www.vintage-radio.net/forum/index.php
Its forum post number is 1254716. Numbering may not have started from one and some posts will have been deleted or merged.
https://www.vintage-radio.net/forum/...16&postcount=9
560 more posts are needed to reach 1 million. 1254716 + 560 is 1255176, so post 1255276 will be the one millionth assuming no deletions or mergers.