Ask exploding heads

I think the earth can support many times it's currently population. However; we need to build infrastructure for that to happen. We can build cities under the ocean, we could even farm underwater and that would be easier and cheaper than building a colony on mars. There is a ton of untapped energy on earth. The Sahara Desert alone gets enough sunlight to power every household in the world. It's just a matter of transporting and storing that energy.

People could live underground and harnes the geothermal energy of the earth.

While we are doing these things we can attempt to start colonies on mars and in the clouds of Venus.