Sure. You can cram another 15 or 20 billion people in here. Maybe even more.
Once you've done that, how do you plan to feed them? Remember, food takes room to grow. If you blasted every mountain flat, filled in every canyon and gully, and turned the entire planet's arable land surface into farmland...you MIGHT have enough farmland to grow subsistence crops to feed them, and provided you also turn the oceans into massive protein farms. Forget livestock. Forget meat. We won't have the room.
But then, what do you plan to give them to drink? Remember that fresh water was a limited commodity. It's not as easy to come by as you think. And it takes water to grow crops--and lots and lots of it. Our farms are sucking aquifers dry NOW. Not to mention you have to drink it, bathe with it, wash clothes with it, and what about the water to support industries capable of sustaining a civilization of 15-20 billion people...or more? And desalinization is expensive, both in terms of money and energy.
Clothing and housing take energy to make and raw materials. For that matter, so does agriculture. Fertilizers have to come from somewhere. Right now, we make quite a bit from oil. And it takes energy to run farm the machinery to raise the crops that 20 billion people need to survive. Right now, that means MORE oil. Whether or not you believe that we're nearing Peak production now or not...the oil supply is *finite.* God knows if we'll be able to wean off oil and onto something else--assuming it's even possible, assuming we can somehow yank a technological rabbit out of our asses--before Peak Oil and the Malthusian die-off finally catches up with us. And that's with our present population.
So yeah, you can cram a few more tens of billions of people onto this bitch, if you don't care about their standard of living at all, don't care about providing medical services, don't care if they're eating a diet that consists entirely of boiled grain and water with an algae chaser or that 95% of them are dirt-ass poor, and don't care that you've turned every last inch of remotely arable land on the entire planet into one solid farm. We can turn every city on earth into Calcutta. For a little while.
Until you finally suck the planet dry of every last source of energy and raw materials, and completely crash the biosphere. And then you'll see what happens when 20 billion people have no clean water or food, and every last form of life on earth except maybe cockroaches, flies and bacteria are dying out.
And the living will envy the dead.
