So interesting thing about population of Rome is that one group of archeologists says it was around 450000. And another thnks it was between 750,000 and a million.Rome had 1 million people who were kept fed by Egypt and the entire north African coast, thanks to the Roman Empire having complete control over them.
And Waterdeep has 2 million inhabitants.
The penninsula that is now italy easily fed Rome and provided exports.
Now at its height the Roman empire I s estimated to have between 45 million to 120 million. (Again experts dont agree) thats who all those other farming regions fed.
Farming has so many variables. the Aztechs fed 5 million people in a much smaller area with much poorer soil and water access.
The population of japan in 1300 10 million
1600. 22 million. And they were able to feed themselves and export rice.