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.
The gzillions of other copies of you are not relevant unless they exist in universes exactly like yours from your observational perspective.
That being said, your point is interesting but just gets back to a core problem of the SA itself, which is how you count up the set of probable universes and properly weight them.
I think the correct approach is to project into the future of your multiverse, counting future worldlines that could simulate your current existence weighted by their probability.
So if it's just one AI in a box and he doesn't have much computing power you shouldn't take him very seriously, but if it looks like this AI is going to win and control the future then you should take it seriously.