After consulting Google, I can summarize the population of three countries. As a Senator, I would not expect to know all these numbers, yet as Cruz said something like “I don’t know” he left it wide open to be asked, “You don’t know the population of a country you want to (go to war with - something like that)”
Country - 1970 - 2020
Iran -28.0M - 83,9M
China - 827M - 1.41B
USA - 203.3M - 331.4M
So a decent answer would be “lots”. Yet in the case of Iran, they have a rather large standing army, and air force and first you have to get over their mountains if you want to invade them with infantry. Yet this was before all those bunker-busters were victorious.
Dunno about invading or bombing China. Neither seems a good idea. But to say they have more than a billion would sound smart for a senator from Texas.
Same with the USA. “I recall, I think it was Walter Cronkite, who said there were two hundred million people in 1977.” and that there are likely more now.
(all numbers approximate and I’ve no idea how they were arrived at except the USA - as I was part of the 2010 Census I can unequivocally say there was a Census)
ETA: I wanted to add Russia, as I know that country proper, is losing population bigly and not just due to people fleeing (or perishing in) Putin’s war. And I wouldn’t believe any of the “census” numbers, and indeed, the male population has taken a staggering fall not unlike WWII yet one they cannot sustain now.