Dumbledore wonders if one of the remaining two Horcruxes belonged to Ravenclaw, since Gryffindor's only known relic, the sword, is safe in Dumbledore's office. Dumbledore believes that Nagini, the snake, is the sixth.
Dumbledore believes Voldemort chose Harry because Harry was a half-blood, just like Voldemort.
Dumbledore then sees Riddle's conversation with Slughorn where he asks the professor what would happen if a person split his soul seven times. As the other two answers say so ably, we know that Dumbledore knew that Voldemort intended to make 6, splitting his soul into 7 pieces.
Dumbledore gets confirmation that Voldemort did indeed make a Horcrux, but alarm bells go off in his head. If Voldemort treated his Horcrux as expendable, that means that Voldemort must have made more than one.
He does not find out when he realizes Harry's scar first starts hurting him. Dumbledore and Harry do not really talk about the pain Harry experiences in his scar until the later books, but it's sort of generally accepted that Dumbledore knows about it.
Dumbledore knew that destroying Voldemort's horcruxes was the only way to truly defeat him. He believed that Harry was the one person who was smart, courageous, and perhaps lucky enough to find and eliminate all the horcruxes.
Harry Potter and Dumbledore recovered this locket from a seaside cave at great cost, weakening Dumbledore hours before his murder, only to discover that it was a fake.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
Voldemort only ever intended to make six Horcrux, with the possibility that maybe he could create more in the future. However, when Voldemort attempted to kill baby Harry that fateful night, his killing curse backfired instead.
“Dumbledore didn't want to lose his wand at that point and Draco disarmed him. So that meant that the wand gave Draco its allegiance, even though Draco never knew it, even though Draco never touched it. “From that moment on, that wand gave its allegiance to Draco, and it wouldn't work as well for anyone but Draco.”
The Dark Lord — aka Voldemort — created seven Horcruxes in an attempt to sustain immortality.
Although Voldemort had six true Horcruxes (not including Harry) in total, no more than five existed at one time because T. M. Riddle's Diary was destroyed before Nagini was turned into a Horcrux.
First of all, Dumbledore didn't reveal the secret of horcruxes to Snape because he didn't find it necessary. It wasn't his style to let out unnecessary secrets to people. Making horcruxes to fight horcruxes wasn't the plan.
However, near the climax of the book, Snape leaves Harry his dying thoughts (to be viewed in the Pensieve) and ultimately reveals to Harry that he had been loyal to Albus Dumbledore throughout the series.
Dumbledore knew it meant Harry would need to die in order for Voldemort to truly die, so he was pleased to know that Voldemort's mistake in taking Harry's blood might just save Harry's life.
I trust Severus Snape completely. '”
Voldemort wasn't aware of Harry being a pseudo-horcrux because he didn't plan it, and Harry didn't know either until Voldemort “killed” him, but he actually killed the piece of soul kept in him.
So when Ron said three, it wasn't a mistake. He meant there were three more horcruxes to destroy before being able to destroy the fourth and final piece which was in fact Voldemort himself.
The Killing Curse does not involve flinging pieces of one's soul around.
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
Her sacrifice gave Harry the power of love, a protection spell sealed with the blood spilled by her death.
Question number one: Why didn't Dumbledore just give the sword of Gryffindor to Harry instead of leaving it to him in his will? He knew even before his death that Harry would need the sword to destroy Horcruxes, and he must have known that the Ministry would not let Harry get the sword.
'Master Regulus told Kreacher to come back,' he repeated. Though he had trodden too far down a dark path, Regulus was still a smart guy. After all, he was the one who deduced that the locket had become a Horcrux, knew it meant Voldemort was no longer 'mortal' and that it had to be destroyed.
Dumbledore knew for sure that the cave is going to be very dangerous, but he didn't know if he and Harry will be able to get the horcrux. So if they had carried the sword with them, they would have taken a great risk.