There is no way to see member numbers by country (or the flag the member chose for themselves).
You can look at rating pools in the stats, filter by country and get some numbers, that will show you some but that is limited to active players and isn't going to give accurate counts; only 350 blitz players from Japan were listed as active.
There are over 100 pages of members from Japan (10 per page) listed on v2 on a member search and the blitz numbers there are 4,896 players that are listed as blitz players there. That won't count any members that haven't played any blitz games, of course.
Is there any page listing number of accounts by country ? How about their increases over time? I'd like to know how many use/used Chess.com from Japan.