My question is how did they determine the numbers back at
the beginning when there were no true world communications?
Lots of sources out there, but here's an interesting one that discusses one way the number of deaths was estimated for the Black Death: https://quod.lib.umich.edu/f/frag/9772151.0006.001/--black-death-bodies?rgn=main;view=fulltext. Scroll down to the section titled: How Many the Black Death Killed.
Basically, they estimated the number by examining tax records. That's right, you had to pay a tax when someone in the household died!