Climate change and overpopulation

There are several “elephants in the room” when it comes to climate change. One of them is population overload. Last month, the world's population was believed to have reached over 6.6 billion. Our planet’s population continues to grow at rates that were unprecedented before the 20th century and today, it’s expected to reach over 9 billion by the year 2050. Don’t try to tell me that our planet overcrowding has nothing to do with climate change; every household in the developing world wants two cars, air conditioning and the rest; just do the math... By definition, overpopulation occurs when the population density is so great as to actually cause an impaired quality of life, serious environmental degradation, or long-term shortages of essential goods and services. So far, certain countries have managed to temporarily accommodate their growing populations through the use of agriculture, desalination, and nuclear power. These technologies however, see their effect decrease over the long-term unless they are sustainable. In that debate, very little of course is said about religion, the second elephant in the room that along with ignorance, the third one, are the leading causes of overpopulation. This begs the question what is a reasonable number for world population?
Is it 3 or 4 billion? Or is 6.6 billion just fine and 9 billion okay?
I’d have a tendency to vouch for the former set of numbers, pushing for less religion and more education… What’s your vote?