When the world hit 10 billion, you will have a legit problem. Food. Where will the food come from?
Pollution cleanup will not help with the food supply.
Because of all these, i am beginning to believe 500million is all the earth need. Just like what was written on the Stone in Georgia.
If you include seasteading, along with pollution clean-up, we are looking at a world that can handle 100 billion.

You can grow all your own food in your apartment using modern techniques. We don't really need farmland. The oceans can be made to grow more than enough food to feed 100 billion. We have the technology to build apartments many levels down as well as many levels up. Living on the oceans and making submersed apartments on floating islands, makes it so that we could actually go far beyond 100 billion. And that is without the global warming it would take to grow more plants and open up Northern Canada, Siberia, Antarctica, the Sahara and other deserts, for habitation and farmland.

If you take into account the warming, it is not exactly a positive impact on living conditions. With the complete thawing of glaciers, the level of the world ocean will increase by 60 meters, which will mark the coastal zone of the continents. The amount of fresh water in the world will also decrease significantly. The number of natural disasters will increase significantly. Hurricanes, typhoons and tornadoes will take thousands of lives, which will complicate the construction of buildings on the water mirror.