The world population refers to the entire number of people of all ages, living in all countries throughout the world. The world population is growing, and in 2015 is approximately 7.3 billion people. The population is growing slowly in OECD countries, which tend to be the wealthy countries that use a great deal of energy. Countries that are not part of OECD (non-OECD countries) have populations that are growing much faster, and make up the bulk of the world's population (roughly 6 of the 7.3 billion).
Fertility rates play a fairly major role in determining how population will grow and have been dropping to close to replacement rates. Many factors are leading to these dropping rates. There's a video at the bottom of this page showing why population is continuing to grow despite this drop in fertility rate.
The world population will continue to grow, and age, but it seems likely that the world population will level off at about 10 or 11 billion people, and possibly drop. This will increase the need for energy, but the far greater driver of the need for energy is the increased wealth that these non-OECD countries are gaining.
Slightly more than half of the world's population lives in cities. These urban populations (people living in cities) are expected to grow, adding 2.5 billion people by 2050. Rural populations are expected to stay the same or even decline. This means that in the coming decades the number of people living in cities will almost double, while the rural populations will remain roughly constant. This urbanization will lead to a dramatic increase in metal use, like steel for construction, as well as increased levels of pollution, increased cost and difficulty of providing public transportation, and destruction of wildlife habitat.
In recent history the world population has gone from 3 billion people in 1950 to 7.3 billion people in 2015 and will continue to grow to 10, or possibly 11, billion people in the coming decades, but is unlikely to grow further. The population is shifting to cities. The increased population will increase the demand for energy, but the increasing average energy use per person in rapidly developing economies will have a much bigger impact on future energy demand worldwide.
While the entire video below from Dr. Hans Rosling is worth watching, the last piece of this TED talk is the most relevant to this discussion:
For more information on the world's population, please see the United Nations population division's website.