All you need to know about this subject you can learn by watching idiocracy. Stupid people are breeding more frequently, and with more partners at a younger age. Smart people are having fewer kids and waiting for their careers to be established. Eventually, everyone is a moron.
But seriously, the world is heading towards overpopulation. Can we say that it is all the fault of china and india? Not so much. Traditions and religion do have a large responsibility in this crisis. If your grandmother had 10 kids and five of them died before they were 3, then your mother might attempt the same thing. But modern medicine has dramatically reduced infant mortality. In your grandmothers time she probably lived on a farm and each extra family member increased the farm output. Modern technology has led to the mechanization of much of the farm work. Coupled with the dream of a better life in the city, your mother probably moved to the city with your dad to seek employment where they could make money easier and faster.
Here is a scary statistic. Most of us will live to see the world hit 11 billion people. But the real question is where will those people be located? We know that a large chunk of them will come from the developing world. We also know that there will not be enough food to feed all of them. Will the wars of the future be over food rather than energy? And on the topic of energy, how will we power our cities for the new demand without completely polluting our environment?
I believe that countries will eventually start closing off their borders. Freedoms will be suppressed for the greater good of the state. Then the ultra-mobs will come. Hungry people armed with whatever they can get their hands on. Millions of them. Like locusts, they will move across the world in a wave, devouring everything in their path. Anybody who is in their way will be torn apart and eaten. Governments that are not ready for this will be toppled easily. Religious freaks on both sides will be calling it the end times and believing that their cause is justified. In the end, only chaos will triumph.
So, to answer the question: anything over 2.17 is too many.