Dumbledore found it rather amusing that Horace had disguised himself as an armchair in an attempt to deceive him.
If the visitor had been a Death Eater, Horace's strategy might actually have fooled them.
Unfortunately, the odds of someone coming to see Horace in the middle of the night being a Death Eater were truly low. Even Horace himself knew that.
Dumbledore was not surprised that Harry had successfully persuaded Slughorn to return to Hogwarts to teach. His old friend clearly would not pass up the chance to add the Boy Who Lived to his collection.
Besides, for an old man who wanted a peaceful life, constantly moving house was simply too exhausting.
Once Horace agreed to teach at Hogwarts, half of Dumbledore's plan had succeeded. The rest was merely a matter of time—getting Horace to reveal how many Horcruxes Voldemort had made.
As for the number of Horcruxes Albert had predicted, Dumbledore was actually willing to believe it. But he could not take such a risk; he still needed to obtain that memory from Horace and confirm it again.
He believed Harry would certainly complete the task well.
The Horcruxes already destroyed were the diadem, the diary, and a venomous snake. That meant there might only be three Horcruxes left, along with Voldemort's main soul.
As for the fragment in Harry's forehead, Dumbledore had already made arrangements.
As long as he destroyed the ring and found the Horcrux Albert had predicted, they would truly be only one step away from eliminating Voldemort completely.
Dumbledore had never been so optimistic about the future.
Once again, he used the Pensieve to revisit Bob Ogden's memory. This was far from the first time Dumbledore had done so; since acquiring the memory, he had lost count of how many times he had watched