I took this quiz after the "First (modern-day) Countries to Reach 5 Million". Indonesia didn't make it, but the last 2 countries that made it didn't get to 5 million until 1600, while this quiz says in 1500 Indonesia was already at 10 million. Which is it?
Iran isn't missing. In pre-modern times, population levels often went down, especially when Steppe nomads rolled through your territory. Fun fact: The population of Iran was lower in 1500 AD than it was in 1 AD.
The demography of England does not include the demography of the other constituent countries of the modern day UK. Might not make a big difference, but worth remembering that the UK and England are not the same thing.
Probably more importantly, once France had decisively won the 100 years' war, England was protected by her relative irrelevance to French interests. France's main foreign preoccupations would I imagine have been with countries like Spain and Austria, rather than relatively insignificant England. I don't think - and I could be wrong, somebody correct me - French kings and governments have ever shown any significant enthusiasm for invading England.
and Louis VIII of France at that point heir apparent to the French throne, who invaded England and was crowned King Louis I, king of England on 21 May 1216. However after the death of King John later that year, most English nobles preferred his 9-year-old son Henry, as he was probably a bit easier to control than the heir to the Kingdom of France
The taxation system of the Kongo Kingdom would have meant an estimate was possible. Given taxation was head tax on each village, and that we have accounts of the finances of the Kingdom at the time, the population can be extrapolated from taxation revenues. Any such estimate would be imprecise, but more likely falls on the low rather than high side.
We have to remember that many of the third world countries of today used to have pretty well formed social & authority structures before everything went sideways during the colonial era.
Very apt name Mx Rome, and yes civilisations from a very early time have had well-formed social & authority structures. That's how they controlled and exploited the masses. It didn't stop with the colonial era, and is yet to stop...
I strongly suspect Colombia would have had more than 4 million in 1500. The Muisca Confederation had a population over 2 million at about that time and occupied only about 2% of the land area of modern Colombia. Inca territories spilled over into Colombia and many other nations such as the Tairona and Muso would have had significant populations - in the latter's case to the extent that they presented a military threat to the Muisca. However, I can't find a source to back up my suspicions.
With 18/19 guessed and one second to go I thought "oh god, it's going to be Bangladesh" and what do you know, it was Bangladesh. 2 seconds earlier and I would have had it!
Iran would make it on 500, 1000, or 1500 years before this quiz. And after this date, especially after 1900 or so, it would start to catch up again. But in 1500 it had fallen behind.
The website you put as the source estimates the world population to be around 426 million but the sum of the populations here is 496 million alone. I think the populations of China and India are way overestimated, for India it's a bit difficult but estimates for Ming dynasty population c. 1500 is ~110 million
see:
https://en.wikipedia.org/wiki/Louis_VIII_of_France#Pretender_to_the_English_throne
or:
https://royalhistsoc.org/calendar/the-forgotten-invasion-the-english-expedition-of-louis-of-france-1215-1217-in-its-european-context/