Well, duh! We hardly know the population of any country more that a few hundred years ago. Even now, some numbers are guesswork - how many Russians have died in the war in Ukraine? That's already an uncertainty of tens of thousands.

For older societies, who even counted as persons? Slaves? Babies? Prisoners of war? Outlaws living in the woods?

And that's even assuming there _is_ a census. The Domesday Book was quite exceptional a thousand years ago, and even that only gives us a population estimate between 1.2 and 1.6 million (see Wikipedia). Any place with a poorer census or written record (i.e most of the world at the time) would be even more of a guesswork.