How many people can the earth support?

this is the most question it get's to my head and I will know .

The US Census Bureau estimated the world population to be 6.94 billion in July of 2011. The rate of increase is currently declining and is expected to level off at about 10 billion in 2050 due to education about birth control and increasing difficulty in supporting larger populations.

Rob already said how many we are most likely to be on earth, so I´m going to say something about limits and reasons and factors.

A important factor in this question is Phosphorus. It is used to manure our crop fields, so we have plenty of food growing. Without it we would have famines more often, even in Europe and the USA. The problem is we need very much of it and the easily accessible deposits will run out over time, its is possible to re-extract it in clarification plants but thats more expensive, so maybe we will get some sort of Peak-Phosphorus, like Peak-Oil. If you wanna know more you may read the the Phosphorus and the Phosphoruscycle article on wikipedia. And don`t worry to much, the human species is tough.

The absolute number of humans the earth can support also depends on how these people live. For example the earth can support 250 times more people if they life like Ethiopians than if the life like citizens of the USA. But living like an Ethiopians may mean you die because you cut your finger with an rusty knife, so we may have to make compromises in our way of life.

Some people may also claim very high numbers supportable if we all become vegetarians, but thats a load of bullshit because we would most likely die from lack of protein especially high grade protein.

Edit: Okay I admit exaggerated above, we won't necessarily die.
But I still think we would get some serious problems suppling the whole humanity with an healthy plant based diet.

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According to me the Earth is already overburdened and cannot support more than 2-4 billion people. Even that is too many people. Especially people who live unnaturally, i.e. wear clothes, use inefficient machines, fossil fuels, cut down trees, shit in drinking water etc.