harry potter - How many people knew the Chamber of Secrets existed?

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

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.