Between 8 and 9 billion. But "impossible" means a chance of zero, and 8 billion / 17,000,000,000,000,000,000,000,000,000,000 > 0, so it's not impossible. The chances that he's lying or delusional are vastly higher of course, but that's no reason to use "impossible" incorrectly.

Impossible is almost always a colloquialism, almost everything is possible is you accept a low enough probability of success. We are talking about something less likely than almost anything else ever called impossible.

No, I think you are misunderstanding. IQ does not describe the likelihood of someone being that smart. It just means you order a number of people by their „intelligence“, the one in the middle is defined as 100 and then it depends on how many other people are in that line which IQ number the person at the end of the line gets. So it’s impossible because the definition of IQ is such that a certain number doesn’t come up without a certain number of measurements.

It‘s as if you would say 150% of all people are female. That is impossible, not just unlikely.