You can calculate a number from the resource (over)usage.
Some will, of course, claim that we consume too much – but at that point, it's a matter of opinion. I call it overpopulation for the current level of technology.
That's right. Overpopulation is a function of technology, etc. More people are coming out of poverty, and according to Bill Gates there will be almost no poor countries by 2035:
If we can't figure out how to generate electricity by means other than coal, for example, we will have a problem. However, technology is increasing rapidly and increasing the quality of life for the majority of people is happening on a large scale.
What number would it have to reach before you concede it is?