How many megacities (those with 5 million or more inhabitants) were there in 1950, and how many do estimates suggest there will be by 2015?

In 1950, only two mega-cities were there, and the estimate suggests that there will be 22 mega-cities in 2015.

Megacities are defined now as the cities having 10 million or more than 10 million inhabitants.

There are 37 mega-cities now in the world, which include Tokyo and Greater Jakarta, these cities have population more than 30 million inhabitants.