What you are describing is something called a population “Critical mass”!
This is the maximum number of people the Earth can support before all the food, water, energy and space runs out. If the population gets above this number people will start to starve and run out of space.
If we run out of energy sources this means big problems for humanity as a whole! This is why people are trying to recycle more, use less fuel and eat less food.
Either people need to have less children or stop using so many resources!
Although technically we’ll never run out of energy sources as long as the sun is still shining – if we could harness the sun’s energy efficiently we’d have more than enough energy for a very long time… plus there are renewable energy sources like wind & wave energy.
BUT energy isn’t much good if we don’t have the space to grow anything or a toxic atmosphere. It seems likely the next big crisis in resources will be fresh water – because there is only a limited amount of fresh water in the world and everyone needs it to survive!! Even taking the salt of sea water won’t be enough…
To combat this, we need to use far less water (in the west) (eg a large amount of california’s fresh water is used to keep golf courses green!!!) , and to be much better at recycling waste water…