This is a very nice apartment with plenty of space (for Tokyo) in a convenient part of the city. Tia is an incredibly friendly, communicative, and helpful host. She provides instructions and recommendations on getting around, places to go/see, and is very prompt with any questions and requests.
This is a great place to stay for 3 people and completely acceptable for 4 (with a fourth going on the futon in the living room). I would be leery of any more than that, with the beds being quite small to share. But if you're cool with sleeping cozy, 5-6 could stay here.
The apartment is clean, comfortable, and well-furnished, with a good complement of amenities by Tokyo/Japan standards. While it is in Shinjuku, it's in a quiet residential area about 10-15 minutes away from the hustle and bustle associated with the area.