I don't post here often - so don't flame me if this has been posted before :/
But I found the answer in most links to be 9-10 billion - due to limited land, water etc.
So if it's taken 12 years for the last 1 billion .... does that mean that life will become quite tough in 36 years (or less) when we hit 10 billion?
Cohen argues that you could fit one billion people each a metre apart, into a field 32km square. So everybody in the world would fit easily into Yorkshire. But it takes 900 tonnes of water to grow a tonne of wheat, and there is only so much water, so much land and so much sunshine. Human action has its own "ecological footprint"; there has to be so much land to provide food, clothing, shelter, medicines, building material, fresh air and clean water for any one human. It takes, according to some calculations, 2.1 hectares of land and water to provide for one average human. The important word is: average. The American footprint is about 10 hectares. So if all humans lived at US standards, we'd need another four Earths.