It's not about the number of humans. It's about the number of observer-moments. Imagine if you were the only human ever. If you're only twenty years old, it's unlikely that you'd live to be a billion. You're not going to just happen to be in one of the first twenty years.
What does that mean, "You're not going to just happen to be in one of the first twenty years"? There are people who have survived more than one billion seconds past their twenty first birthdays. And each one, at one point, was within twenty second of their twenty first birthday. What would you say to someone whose twenty first birthday was less than twenty seconds ago who says "I'm not going to just happen to be in the first twenty seconds"?
A self-modifying AI is built to serve humanity. The builders know, of course, that this is much riskier than it seems, because its success would render their own observations extremely rare. To solve the problem, they direct the AI to create billions of simulated humanities in the hope that this will serve as a Schelling point to them, and make their own universe almost certainly simulated.
Plausible?