I have tried to read up on the dragons of the sixth world as much as possible, but I there are some mysteries that make things unclear.
1. Are all the Great Dragons in the world known by the metahumans of the Sixth World and accounted for, or are some Great Dragons only known in the deepest Shadows?
3. When the dragons gather, as they have when the title of Loremaster was contested, do they do a headcount and find some lesser and greater dragons unaccounted for since the Fourth Age?
Hestaby's thoughts in "Survival of the Fittest" fluff makes it clear that some don't show up to the meeting & that they have not been heard from in the 6th age. IIRC, She is unsure if they were killed in sleep, have not awoken yet, or did wake & have just been lying low.
Given the above, I'd say its quite likely that humanity does NOT know about every Great Dragon at this time.