In 2008, there was the rule of 6 flights for a round trip but I managed to book 4 flights outbound and 2 inbound (something like NYC CDG CPH (stopover) CDG TYO (destination) CDG NYC). It took a bit of persuasion but at that time it was written nowhere that the flight distribution needed to be even, max 3 per direction. Not sure how it stands now. So if you return with only 2 flights, you might be able to force your luck for the outbound.