The property is quite spacious. But I think the suggestion of 16 guests would be too many people for the amenities available i.e. 1 shower room (with 2 shower heads). The description is misleading when it says it has 4 bathrooms. It should have stated 4 separate toilets and 1 shower room (with 2 shower heads) and 3 sinks. Unless you are a couple then you wouldn't be using the shower together. We had 5 guests for the stay.
The property is more suitable and nicer for the warmer months of the year i.e. Spring through to Autumn as the heating available for the winter is inadequate. The hallway will be very cold so keep the doors closed to conserve the heat. There was floor heating in the living room.
The issues we experienced is upon check-in where there was no key provided to enter or lock the property and the property was left unlocked. This was resolved in the end but not without inconvenience. They only provided 1-key for the whole apartment so you will have to co-ordinate if you are all coming back separately or try asking the host for more keys.
2 of the 5 bedrooms could not be used as the heating would not work. Our stay was around the end of January 2018 - I guess this didn't help as it was the coldest winter in Tokyo for 45 years.
Overall the place is maintained to a reasonable level with basic provisions covered i.e. towels and soap only. 8 mins walk to Nishi-shinjuku-gochome and 12-15 mins taxi to Shinjuku or 30 min walk. Family mart nearby and 24 fast food place down the road.