Well, let's try to be a bit more specific about this.

First, what does the claim that "the world is overpopulated" mean? It implies a metric of some sort to which we can point and say "this is too high", "this is too low", "this is just right". I am not sure what this metric might be.

The simplest metric used in biology is an imminent population crash -- if the current count of some critters in an ecosystem is pretty sure to rapidly contract soon we'd probably speak of overpopulation. That doesn't seem to be the case with respect to humans now.

Second, the overpopulation claim is necessarily conditional on a specific level of technology. It is pretty clear that the XXI technology can successfully sustain more people than, say, the pre-industrial technology. One implication is that future technological progress is likely to change whatever number we consider to be the sustainable carrying capacity of Earth now.

Third, and here things get a bit controversial, it all depends (as usual) on your terminal goals. If your wish is for peace and comfort of Mother Gaia, well, pretty much any number of humans is overpopulation. But let's take a common (though by no means universal) goal of long-term economic wealth. We want to create value and keep on creating more of it for a long time. Given this, you want more humans since that will accelerate the process up until certain limits. Where these limits are is debatable but I haven't seen much evidence that we are facing them right now.

Fourth, overpopulation is pretty local. Taking the simplest possible measure of land area, it's hard to argue that countries like Russia or Canada or Australia are overpopulated.