I’ve been looking for the details on that specific topic since last June (sad 
)
The closest I know to a “number” - if we consider (very generously), every not anti-Calia forsaken to be automatically pre-Cata forsaken (and in such case almost guaranteed - lordaeronian) it is
the number of Forsaken who may welcome their presence is not fixed
So, any logical attempts to analyze what happened to population, and “tracking” migration of the scourge to find out how many and in what regiongs then became the forsaken, will still bash into numbers popping out of thin air for whatever direction the narrative will take IMO.
gl hf