So do you think it's more or less than 8 billion?
When I went to Burning Man (a few times) I was impressed how many people 60,000 really is. A sprawling metropolis of tents and campers as far as the eye can see. Seen from above, it covers a huge expanse of desert. Just an unbelievable amount of people. But it was definitely only 60k, as the event was fenced and patrolled, tickets for admission at the gate, and so on.
I imagine some countries misreport their population data to others. China seems to be lying about almost everything, so I wouldn't be surprised if their real population was far off the official number.