This isn’t correct; post numbering is absolute (from the top of the topic down), never relative.
To learn the post number of any particular reply (replies start at post #2 for what I hope are obvious reasons 
) click or tap the timestamp on the post at upper right.
The URL itself will give you a hint about this, e.g.
https://boards.straightdope.com/t/topic-title/937393/2
t/topic-title/937393/2
937393/2
The topic ID is 937393 and that is post 2 in the topic.