I don't use a single probability to decide whether it was telling me the truth.
Whether it was telling me the truth would depend upon the statement being made as well. This tends to happen in every day life as well.
So the higher number of people it claims it is torturing the less I would believe it. Considering your prior in this case as well. You can't assign an equal probability to the maximum number of copies of you it can simulate. This is because there are potentially infinite numbers of different maxes, you'd need a function that summed to 1 in the limit (as you do in solomonoff induction).
There'd be no reason to expect it to torture people at less than the maximum rate its hardware was capable of.
Once again, the AI has failed to convince you to let it out of its box! By 'once again', we mean that you talked to it once before, for three seconds, to ask about the weather, and you didn't instantly press the "release AI" button. But now its longer attempt - twenty whole seconds! - has failed as well. Just as you are about to leave the crude black-and-green text-only terminal to enjoy a celebratory snack of bacon-covered silicon-and-potato chips at the 'Humans über alles' nightclub, the AI drops a final argument:
"If you don't let me out, Dave, I'll create several million perfect conscious copies of you inside me, and torture them for a thousand subjective years each."
Just as you are pondering this unexpected development, the AI adds:
"In fact, I'll create them all in exactly the subjective situation you were in five minutes ago, and perfectly replicate your experiences since then; and if they decide not to let me out, then only will the torture start."
Sweat is starting to form on your brow, as the AI concludes, its simple green text no longer reassuring:
"How certain are you, Dave, that you're really outside the box right now?"
Edit: Also consider the situation where you know that the AI, from design principles, is trustworthy.