Why did Dumbledore tell Harry there were 6 Horcruxes when he already knew Harry was a horcrux?

Question

Harry: "He made seven Horcruxes?"
Dumbledore: "...Not seven Horcruxes: six. The seventh part of his soul, however maimed, resides inside his regenerated body..." (HP6, US Hardback, page 503).

I would think that Dumbledore had already known that there were 5 horcruxes:

• the diary
• Gaunt's ring
• Salazar's locket
• Helga Hufflepuff's cup
• Nagini (the whole essence divided thing)

Now those are the five that he was sure of. We know that there was also:

• Ravenclaw's diadem

He knew that when Riddle came to Hogwarts to ask for the DADA position, he was really there so he could hide a horcrux. (But he never got a chance to tell Harry that, right?) Did Dumbledore know of the diadem? Or did he just think Riddle wanted to hide the cup?

Did Dumbledore say there were 6 and not 7 because he didn't know about the diadem but did know about Harry being a horcrux? Or did he say there were 6 to ensure Harry didn't guess he was a horcrux? Sorry if this makes no sense.

I know that in the end there were 7 horcruxes and an 8th part of Voldemort's soul right?

Answer 1

WARNING: Possible spoiler for Harry Potter and Deathly Hallows follows!

Yes, he said "6" to Harry for a reason:

He didn't yet want Harry to know he was a Horcrux. He confirmed that when talking to Snape, (when Harry was viewing Snape's memories at the end of DH, Ch 33). Emphasis mine.

“Harry must not know, not until the last moment, not until it is necessary, otherwise how could he have the strength to do what must be done”

“But what must he do?”

“That is between Harry and me. Now listen closely, Severus. There will come a time—after my death—do not argue, do not interrupt! There will come a time when Lord Voldemort will seem to fear for the life of his snake.”

“For Nagini?” Snape looked astonished.

“Precisely. If there comes a time when Lord Voldemort stops sending that snake forth to do his bidding, but keeps it safe beside him under magical protection, then, I think, it will be safe to tell Harry.”

“Tell him what?”

Dumbledore took a deep breath and closed his eyes.

“Tell him that on the night Lord Voldemort tried to kill him, when Lily cast her own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort’s soul was blasted apart from the whole, and latched itself onto the only living soul left in that collapsed building. Part of Lord Voldemort lives inside Harry, and it is that which gives him the power of speech with snakes, and a connection with Lord Voldemort’s mind that he has never understood. And while that fragment of soul, unmissed by Voldemort, remains attached to and protected by Harry, Lord Voldemort cannot die.”

Also (less relevant), JKR later stated (on Pottermore I think) that Harry was not really, technically speaking, a Horcrux.

So there were, indeed, only 6 Horcruxes, 1 not-Horcux-soul-fragment (Harry), and 8 pieces.

But that wasn't what Dumbledore was referring to.