Shanghai Population

What is the population of Shanghai? It seems to be a simple question, since the population of any city at a given time is pretty predictable. However, the number for cities in China is not certain, and especially for immigration cities like Shanghai.

Recently when I did research for classified market in other countries, I realized that Denmark only has 5 million people (while its top classified site dbk gets about 60 million USD in revenue) with 129.16/km² density. How about Shanghai?

My typical answer to "Shanghai Population" question is 16 million, including 13 million residence, and 3 million migration/unregistered population. This number seems to have changed when I just looked up this number.

Shanghai Population

It is so hard to calculate exactly how many people in Shanghai. The general way to do it is to treat people stay in the city for more than 6 months also as Shanghai population. This way, the latest report states, that there are 18 million people in this city, among 5 million are people without Shanghai residence, but stay in the city for more than 6 months…

I believe the number will continue to increase. What does it mean when the population is more than 20 million? I just feel that the idea of mega-city does not work. Hope people give an answer during the World Expo – what is a better for better life?