The human population growth curve is exponential. This is extremely parasitic activity. There must be an orderly, significant drop in the rate of human reproduction, or billions will die chaotically in the near future. As you can see in this graph of human population throughout time, naturally, there should be less then 1 billion people on Earth. However, considering our technological abilities, it is my educated guess that our planet could comfortably sustain 3-4 billion humans.
This graph shows that Asia (in dark blue) holds 4 billion of the worlds 6.8 billion people. As an example of how dramatic the problem is, it projects 9 billion by the year 2054.
The death rate decreases significantly as technology increases. With science and chemicals, humans are able to defeat enemies that are too small to see. Bacteria and diseases serve the purpose of preventing any one species from becoming too dominant.