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