[bufferoverflow]

1000 years is a loooong time. It's entirely plausible to have trillions of humans, if we create conditions for that. And not only on Earth.

For that to happen we need almost complete automation, which would provide cheap food, cheap housing, cheap healthcare, and lack of worry about the immediate future.

Even if you can sustain 1% increase in population per year, 1000 years of prosperity will result in

(8 billion)*(1.01^1000) = 167673 billion = 167 trillion people

(1% is roughly the current population growth, but it's slowing down)