At least 18 times
First, let us note that Dumbledore is a very deceptive individual. Thus he undoubtedly has lied many more times than documented in the main series.
"Oh, did he now? And did he tell you everything, was he honest with
you?”
Harry wanted him with all his heart to say “Yes,” but somehow
the simple word would not rise to his lips.
Aberforth seemed to know
what he was thinking.
“I knew my brother, Potter. He learned secrecy at our mother’s knee.
Secrets and lies, that’s how we grew up, and Albus. . . he was a
natural.”
—Harry Potter and the Deathly Hallows
Harry Potter and the Sorcerer's Stone
She threw a sharp, sideways glance at Dumbledore here, as though
hoping he was going to tell her something, but he didn't, so she went
on. "A fine thing it would be if, on the very day YouKnow-Who seems to
have disappeared at last, the Muggles found out about us all. I
suppose he really has gone, Dumbledore?"
"It certainly seems so," said Dumbledore. "We have much to be thankful
for. Would you care for a lemon drop?"
Dumbledore is, at best, being extremely misleading here. He himself believes that Lord Voldemort survives in disembodied form, and is not "really gone."
"It's just astounding... of all the things to stop him... but how in
the name of heaven did Harry survive?"
"We can only guess," said Dumbledore. "We may never know."
Probably a lie. Dumbledore likely knew at this point that Harry survived because of Lily's sacrifice. That is, after all, why he left Harry with the Dursleys.
"Exactly," said Dumbledore, looking very seriously over the top of his
half-moon glasses. "It would be enough to turn any boy's head. Famous
before he can walk and talk! Famous for something he won't even
remember! Can't you see how much better off he'll be, growing up away
from all that until he's ready to take it?"
Dumbledore does not believe that Harry will be better off with the Dursleys; he left Harry there because of the blood protection afforded by Lily.
"What do you see when you look in the mirror?"
"I? I see myself holding a pair of thick, woolen socks."
This is a lie. As indicated in this answer, J.K. Rowling has explicitly confirmed what Dumbledore sees when he looks into the Mirror, and it is not a pair of socks.
Harry Potter and the Chamber of Secrets
Dumbledore was surprisingly honest in this book. I could not find any untruths he told anyone in Chamber of Secrets.
Harry Potter and the Prisoner of Azkaban
Professor Trelawney ignored her. Eyes open again, she looked around
once more and said, "But where is dear Professor Lupin?"
"I'm afraid the poor fellow is ill again," said Dumbledore, indicating
that everybody should start serving themselves. "Most unfortunate that
it should happen on Christmas Day."
Lupin was not ill: he was sitting in his office in the form of a wolf.
"YOU DON'T KNOW POTTER!" shrieked Snape. "HE DID IT, I KNOW HE DID
IT—"
"That will do, Severus," said Dumbledore quietly. "Think
about what you are saying. This door has been locked since I left the
ward ten minutes ago. Madam Pomfrey, have these students left their
beds?"
"Of course not!" said Madam Pomfrey, bristling. "I would have heard
them!"
"Well, there you have it, Severus," said Dumbledore calmly. "Unless
you are suggesting that Harry and Hermione are able to be in two
places at once, I'm afraid I don't see any point in troubling them
further."
While not a direct untruth, this statement nonetheless contains a highly false implication.
Harry Potter and the Goblet of Fire
“Dumbly-dorr must ’ave made a mistake wiz ze line,” said Madame
Maxime, shrugging.
“It is possible, of course,” said Dumbledore politely.
Though Dumbledore is explicitly described as being polite here, this is nonetheless a lie. Not only is Dumbledore almost certainly too skilled to make a mistake with the line, but it was working very well earlier, as illustrated by various underage students sprouting beards.
Harry Potter and the Order of the Phoenix
“Oh, can she tell us about six months’ worth of meetings?” said
Dumbledore, raising his eyebrows. “I was under the impression that she
was merely reporting a meeting tonight.”
Dumbledore was not under that impression: he had no reason to think that Marietta Edgecombe had not been attending meetings all year.
"You?” he whispered, stamping again on his smoldering cloak.
“That’s
right,” said Dumbledore pleasantly.
“You organized this?”
“I did,” said Dumbledore.
“You recruited these students for — for your army?”
“Tonight was supposed to be the first meeting,” said Dumbledore,
nodding. “Merely to see whether they would be interested in joining
me. I see now that it was a mistake to invite Miss Edgecombe, of
course."
Marietta nodded. Fudge looked from her to Dumbledore, his chest
swelling. “Then you have been plotting against me!” he yelled.
“That’s right,” said Dumbledore cheerfully.
Needless to say, despite the name, Dumbledore had nothing to do with the founding of Dumbledore's Army, nor was he plotting against Cornelius Fudge.
“It is time,” he said, “for me to tell you what I should have told you
five years ago, Harry. Please sit down. I am going to tell you
everything."
Despite his equivocating over this in the later books, he emphatically did not tell Harry everything! He omitted any information about Voldemort's Horcruxes, which he was fairly certain about by then. He forgot to tell Harry that Harry himself was a sort of Horcrux, which was also very relevant. He also neglected to mention that Voldemort might be tied to Harry by having used his blood to reconstitute his body. Finally, he did not mention Snape's role in the deaths of Harry's parents.
Harry Potter and the Half-Blood Prince
He had pointed with his injured hand.
"Professor, what happened to your ... ?"
"I have no time to explain now," said Dumbledore. "It is a thrilling
tale, I wish to do it justice."
While the tale may well be thrilling, Dumbledore certainly has plenty of time. He simply does not wish to explain at the moment.
"You said, at the end of last term, you were going to tell me
everything," said Harry. It was hard to keep a note of accusation from
his voice. "Sir," he added.
"And so I did," said Dumbledore placidly. "I told you everything I
know."
This is pretty exactly as false now as when Dumbledore first said it, in Order of the Phoenix.
"Here," said Dumbledore, waving his wand once as he passed her the
piece of paper, "I think this will make everything clear."
If by "make everything clear" Dumbledore means "I'll Confund you" then yes. Otherwise, no.
"You have — inadvertently, I am sure — been using your powers in a way
that is neither taught nor tolerated at our school."
From Dumbledore's previous conversation with Miss Cole, he must be very much aware that Tom Riddle has been using his powers very intentionally, not "inadvertently": using them to, as he says "to frighten, to punish, to control."
"I am glad to see you appreciate the magnitude of the problem," said
Dumbledore calmly. "But firstly, no, Harry, not seven Horcruxes: six.
The seventh part of his soul, however maimed, resides inside his
regenerated body. That was the part of him that lived a spectral
existence for so many years during his exile; without that, he has no
self at all."
This is all lies. Though J.K. Rowling has said that Harry is not technically a Horcrux, Dumbledore nonetheless referred to him as such. Even if we admit the possibility that Dumbledore is speaking only of Horcruxes that Voldemort directly "made," as opposed to accidental ones, he is still talking about where the "seventh part" of Voldemort's soul resides, while knowing full well that there were more than seven.
"But as for being about to kill me, Draco, you have had several long
minutes now. We are quite alone. I am more defenseless than you can
have dreamed of finding me, and still you have not acted ..."
Draco and Dumbledore are not alone. Harry is standing under the Invisibility Cloak mere feet away.
Harry Potter and the Deathly Hallows
This is the book in which so many of Dumbledore's lies and half-truths are revealed. Besides that, he is of course dead and thus cannot lie very much. Nonetheless, there are a few:
“Did you think that breaking the ring would break the curse?”
“Something like that. . . I was delirious, no doubt. . . ” said
Dumbledore. With an effort he straightened himself in his chair.
“Well, really, this makes matters much more straightforward.”
At this point, Snape does not know about Voldemort's Horcruxes. Dumbledore was not trying to break the curse by breaking the ring, he was trying to destroy Voldemort's Horcrux. He also certainly was not delirious.
“So the boy. . . the boy must die?” asked Snape quite calmly.
“And Voldemort himself must do it, Severus. That is essential.”
Dumbledore does not believe that Harry must die. He thinks it very possible that his connection with Voldemort, through Harry's blood, may save the latter from death (as indeed happened).
The Tales of Beedle the Bard
The irony is that a curious legend has grown up around this story,
which precisely contradicts the message of the original. This legend
holds that the gifts Death gives the brothers – an unbeatable wand, a
stone that can bring back the dead, and an Invisibility Cloak that
endures forever – are genuine objects that exist in the real world.
The legend goes further: if any person becomes the rightful owner of
all three, then he or she will become “master of Death”, which has
usually been understood to mean that they will be invulnerable, even
immortal.
We may smile, a little sadly, at what this tells us about human
nature. The kindest interpretation would be: “Hope springs eternal”
In spite of the fact that, according to Beedle, two of the three
objects are highly dangerous, in spite of the clear message that Death
comes for us all in the end, a tiny minority of the wizarding
community persists in believing that Beedle was sending them a coded
message, which is the exact reverse of the one set down in ink, and
that they alone are clever enough to understand it.
Their theory (or perhaps “desperate hope” might be a more accurate
term) is supported by little actual evidence.
Dumbledore personally knows this theory to be absolutely true, and as such is lying to all his witch and wizard readers.
