The official level is: whatever the story teller determines. Kinda disappointing, but there are in-universe reasons why the game doesn't outright state "this is exactly what hunters know".
In the VTM:20 book, it states "Not even all of the Arcanum participants [...] are aware of one another's work." The Arcanum operates in loose pockets and cabals, and although there is an overall organization, the flow of information isn't always completely transparent. One reason for this is when the Arcanum gathered "too much" information on vampires, vampires burned down the Boston Chapter House in 1910 to send a message. After that, there deliberately was a diminished push for centralized information due to fear of reprisal of the investigation subjects. In modern games, this can be handled different since not all information needs to be written down, but that's still up to storyteller discretion.
These two points are what allows storytellers to adjust the amount of information present and given to players. The information is not readily shared between all members, and the Arcanum is reluctant to have a centralized repository of information regarding vampires.
There is also a splat book called The Hunters Hunted that specifically expands the organizations as well as their tactics for dealing with kindred, and it may contain more information that wasn't already recompiled in VtM:20.