One more question, is the summary below for the mother board (Bitcoin Discussion) or the combined mother and child board?
It's the combined posts and replies from all child boards under it. I did a quick calculation;• There are 1349 pages of threads currently and 40 per page, so this would result in about 53,960 posts for Bitcoin discussion board only
• If you add the posts from its 4 child boards you'll get the approximate number circled in your screenshot, so it is most certainly a combination of all boards under it.
I only see this information on child board, example Legal child boards - new posts (Topics:2364, Posts:54381) - but I can't seeing that information on mother board (Bitcoin Discussion), is only me or something happen in my browser?
It appears same way on my browser. The main board is a combination of all other boards under it, probably why there's no distinction when on the bitcointalk landing page. If you subtract the number of the individual boards from the general number, you'll get a rough estimate. 
