system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is world population calculated? And how accurate is the number? Is it possible that the number can be entirely wrong?
gpt
In general, information about the world population is aggregated from information about each country's population.  From there, each country usually either conducts regular censuses or has some form of national registration to track people (or both) as it's a very important part of running a government.

These estimates are only updated so often though (IE, the US conduct it's census only every 10 years), so there is a little math done to the number to get the current estimate.  There are three big factors that matter when creating that current estimate: Mortality rate, Birth rate, and Migration.  The first obviously creates downward pressure on the number, the second upward, and the third can be either depending on if the new flow is out of or into the nation.

How accurate is it?  Well, the UN is one of the better sources, and they estimate their margin of error to within 1% or so for the entire world.  It varies HIGHLY depending on the country though, with less developed nations and especially wartorn nations being incredibly difficult to estimate due to difficulties in conducting censuses, and highly developed nations being very accurate.  Broadly speaking, you can trust the numbers though and there is very little to no chance that the number can be entirely wrong, unless you are getting a number from a shady source.