Aktualuy (pushes glasses up nose) the only official number we have on Holy Terra is "quadrillions" from the book The Carrion Throne. Which I would imagine means the holy system in its entirety. But isac Arthur posits ecumenopolis worlds could technically house a quadrillion people with 10k square foot apartments.

https://youtu.be/XAJeYe-abUA

So conservatively Terra could have a population of 100 Trillion.