Economic philosopher Jeremy Bentham developed the concept of Utility, a quantity roughly commensurate with the human sense of pleasure or goodness, and diminished by pain or badness.
Modern economic theories posit that markets regulate human activity to maximize utility. These theories are full of extremely flawed assumptions and are mostly wrong. (See for example the excellent book, Debunking Economics by Steve Keen.)
Among economists and others there is a notion that more human beings on earth increases utility as well. Since under almost all circumstances, humans would prefer life to death, each additional person increases the total number of humans that would experience positive utility (and hence there is more Utility) unless circumstances get very bad.
I believe this is wrong on many ways, and that a much smaller human population would be better. How is "more Utility from more people" wrong? Let me describe some ways:
1) Not Just Humans. The earth includes many non-human species who also experience pleasure and pain and goodness and badness in other ways. There is no universal reason why their Utility shouldn't count. Increasing human population generally increases the human planetary footprint, reducing the land space, biodiversity, unpolluted water and air, and other things which are needed to support non-human wildlife populations and their Utility. Sure, humans could organize their activities so that they have less impact. But they've tended not to, and even if they did, a larger human population only makes it more difficult to minimize the human ecological footprint.
The unfortunate fact is that the increasing number of Humans has created what is already the 6th largest extinction event in Earth's history, and it is likely to get far far worse over the next few hundred years.
2) Limited Natural Resources and Sinks.
While additional humans means more possibilities to experience Utility, it may diminish the Utility available to others in the cases of limited resources and overcrowding. At some point the latter decreases may be greater than the hoped-for increases. This is obvious when you go from a situation where everyone can easily be fed to one where there isn't quite enough, but occurs in many other ways as well.
4) Wrong model of Utility?
The notion that Utility is nearly automatic to human life may be fundamentally wrong. I describe this idea last because it is somewhat speculative and could be wrong.
My own idea is that humans do not automatically have a positive utility, but rather neutral utility, i.e. zero. It is network effects related to the existence of other human beings, as well as resources and opportunities, that provide either positive or negative utilities.
Such a model of utility arising out of the natural and social networks makes utility highly unstable. When things are good they can be very good, as when there is a Golden Age. When things are bad they can turn very bad, as in world wars or societies based on terror.
This also makes Utility highly contingent, not the sort of thing that can just add up when you have more people. Fundamentally, and not even referencing limited resources, more people can be either positive or negative. It depends.