Almost all hotels in Japan charge per person rather than per room. Most Marriott properties will charge a few thousand yen more for two adults than for just one. Adding a 3rd will boost the price even more (¥5,000 or more).

I had to ask the Lurker to help me out the first time I booked an award for 4 adults and the property wanted to charge for the extra 2 (even though it was a room that accommodates up to five). In subsequent stays the extra charge hasn't appeared on the booking page so I guess they ironed it out.

Elite breakfast has definitely been limited to just two per day.