Yes, I believe many things are the truth, to the extent the systems we have in place to determine these things are capable of measuring them. Do I know how many seats are in Yankee Stadium for baseball games? No, because I've never taken the time to count them all, but I bet you if it isn't 46,537, it isn't very far off it and the reason it isn't correct has no sinister reason.

Nobody knows how many people there are. However just about every nation I am aware of conducts censuses on a periodic basis and I think those are a decent enough approximation to conclude the World's population is much closer to 10 billion than it is to 1 billion. Simply because what benefit would there be to lying about this on a national scale and even if some nations did would it really impact the result much? Does it matter if we have 6 billion or 8 billion people really? It's a shit ton either way.

The question that matters isn't how many people can the Earth support, the question is what kind of life we want tho population to have and that will determine how many people. At some point in a basically closed system it becomes a zero sum game, to add more people means everyone else must give up something.

Or, expressed different, how many people can an Airbus 320 hold? Single class seating it's 180 people, but wouldn't it be much nicer if you got rid of all the middle seats and made it seat 120 instead? Or it seated 80 and you had no middle seats and 33% more legroom?