How many people live in Tokyo prefecture?

How to get close to this number

Take the metropolitan figure and apply a core share. This site's companion question puts the wider agglomeration around thirty-seven million, and a metropolitan core typically holds a third to a half of its region. A third of thirty-seven million lands you very close, and the two questions cross-check each other.

The unit here is a prefecture rather than a city, which is a distinction worth catching. Japanese administrative geography does not map onto the city-and-suburb model, so the entity being counted is a regional government covering both dense central wards and much quieter western areas. Comparing it to a European "city" figure compares different kinds of thing.

The rail statistic is the one that explains how the region functions: around forty million passenger journeys a day, which is more journeys than there are people in the metropolitan area. A city of this density is only possible because the transport system can move its population several times over each day, and that figure is the real constraint on how large it could grow.