Answer:
The more people in the world, the more babies are produced.
However in certain areas of the world, not enough people are being born and so we will not have enough workers in those areas who will be able to help those who could no longer work. In order to keep the same rate of population you need to have on average 2 children per family. There are exceptions, but on average the USA is reproducing at less than 2 children per family currently.