The earth, can hold how many people?

I read somewhere that the Earth has the capacity to host around 20 000 000 000 people. This includes proper feeding and clean drinking water.

Birth rate increasing rapidly. 10 in 1, ten for giving birth and 1 for die. We are facing the problem of pure water and resource. In my country, Government has program to reduce birth rates, it's called Family planning. It mean, every family should only have 2 children. It work although just help slightly to reduce the birth rates

Human population increases more than double from 2.5 billion in 1950 to nearly seven billion. Corresponding to the CIA World Factbook there are around 6.8 billion people living on the world as of July 2009. That number will magnify with about 212,000 every day.