How many people live in the Cairo metropolitan area?

How to get close to this number

Anchor on the country and the concentration. Egypt has over a hundred million people, and almost all of them live along a narrow strip of river valley and delta because the rest is desert. A single metropolitan area in that strip holding a fifth of the national population is entirely plausible, and that gives around twenty million.

The geographic constraint is the whole story. Habitable land in this country is a few percent of its territory, so population density within the inhabited area is among the highest on Earth even though the country looks sparsely populated on a map. Any population question about a desert country is really a question about water.

The new capital in the explanation is the consequence. A government building an entirely new administrative city in the desert is a response to a metropolitan area that has physically run out of room — and it is a solution several countries have tried, with mixed results. Population figures sometimes explain policy decisions that otherwise look inexplicable.