How many people knew the Chamber of Secrets existed?

Question

And of course I am asking about the time before the events of Book 2. Although we don't see Hermione quoting anything directly about the chamber from "Hogwarts: A History", from Prof. Binn's class we get the gist that the chamber is a legend, and nobody really believes it exists, as no one had been able to find it in all of 1000 years of Hogwarts history.

But we know that Voldemort had actually found it, and from the dialogue between Dumbledore and McGonagall, they knew it definitely existed as well.

The chamber of secrets has indeed been reopened

and

the question is not who, the question is how.

As Lucius Malfoy put the diary on Ginny to be taken back to Hogwarts, it seems possible that he knew about it's existence as well, and yet, among the general population it is a myth.

How many people exactly knew that the Chamber of Secrets definitely existed, before the events of book 2?

Answer 1

All of Slytherin's descendants knew, some others did as well

The existence of the Chamber was known to Slytherin’s descendants and those with whom they chose to share the information. Thus the rumour stayed alive through the centuries.
_{Pottermore - Chamber of Secrets}

The only such descendents that we know by name are the Gaunts, or more specifically, Corvinus Gaunt.

The presence in school at the time of a student called Corvinus Gaunt – direct descendant of Slytherin, and antecedent of Tom Riddle – explains how the simple trapdoor was secretly protected, so that those who knew how could still access the entrance to the Chamber even after newfangled plumbing had been placed on top of it.
_{Pottermore - Chamber of Secrets}

Everyone else either heard about it or heard rumour about it from them, starting with people in Slytherin's own generation.

the fact that rumours of its existence persisted for so long reveals that Slytherin spoke of its creation and that others believed him, or else had been permitted, by him, to enter.
_{Pottermore - Chamber of Secrets}

While the secret died out with the Gaunts, some, such as Tom Riddle decided to investigate the rumours. Tom unleashed the monster in the 1940's. Though it was hushed up, we can see that many, including the Hogwarts faculty, were aware of it.

Answer 2

This is a very hard question to answer. It could be only a few if you define knowing that it existed as actually having been there--or it could be a lot more, who were told that it had been opened or heard it as a story.

Let's begin with those who knew it existed, because they were there, before Book 2.

Salazar Slytherin, but not the other three.

The story goes that Slytherin had built a hidden chamber in the castle, of which the other founders knew nothing.

Tom Riddle

He opened it in his fifth year, and again in Book 2, through Ginny.

The Basilisk

The basilisk did live in the Chamber, and therefore I assume knew about its existence.

So those are the people who've been in the Chamber. But wait...there's more!

Aragog and Hagrid

Did Aragog enter the Chamber of Secrets? I don't think so; but he was blamed as the monster and Hagrid as the opener, so they presumably knew that it existed.

Here's something Tom Riddle says to Harry in his diary:

In my fifth year, the Chamber was opened and the monster attacked several students, finally killing one. I caught the person who’d opened the Chamber and he was expelled. But the headmaster, Professor Dippet, ashamed that such a thing had happened at Hogwarts, forbade me to tell the truth. A story was given out that the girl had died in a freak accident. They gave me a nice, shiny, engraved trophy for my trouble and warned me to keep my mouth shut.

That explains why it's not public, but there are more people who knew about it.

Armando Dippet and Hogwarts' faculty, including Dumbledore

They must've known about the Chamber; Dippet was the one who began to conceal it.

Hogwarts students

Here's a quote from the wiki:

Aragog was framed as being the monster and Hagrid was accused of being Slytherin's heir.

You can't frame something like this without an audience; someone must've known about this, and news travels fast among these kids. They may not have known, but they would have suspected. Count them if you want to.

Did Lucius know?

“So — have you stopped the attacks yet?” he sneered. “Have you caught the culprit?”

“We have,” said Dumbledore, with a smile.

“Well?” said Mr. Malfoy sharply. “Who is it?”

“The same person as last time, Lucius,” said Dumbledore. “But this time, Lord Voldemort was acting through somebody else. By means of this diary.”

From this, I doubt that Voldemort would tell him his plan, but he probably knew about it from the rumors.

EDIT: found this quote. It's not proven, and it's only one of Dumbledore's shrewd ideas. On the other hand, those usually turn out to be correct:

I understand that Voldemort had told him[Lucius] the diary would cause the Chamber of Secrets to reopen, because it was cleverly enchanted.

Did Myrtle know?

“I died.”

“How?” said Harry.

“No idea,” said Myrtle in hushed tones. “I just remember seeing a pair of great, big, yellow eyes.

So no, she did not know that it existed, she just died. She might be counted among those Hogwarts students who heard the rumors.

Answer 3

There could be more people than you would expect.

The only people confirmed to know were Aragog, Tom , and Lucius but there could be more depending on who Voldemort told.

The first people that come to mind would be the people Voldemort might told while he was opening it. That would mean his former "gang" at school. Namely Avery Sr. and Lestrange Sr., among the other named people.

Next comes the people who figured it out somehow during the event. Dumbledore, Aragog, and Myrtle all knew something was happening, but only Aragog knew for sure.

Lucius had to have known that the diary opened the chamber, that was what he had intended to happen. That begs the question of other high ranking death eaters knowing (I personally doubt it as he spread his information pretty thinly)