its very hard to give an estimate based on these pics for a few reasons:
we cant actually see the beard hair. you mentioned they are gray,
it seems you may have just recently shaved? growing it out a bit would give us a better understanding
where you put the red arrow is probably too far south for a doctor to take from. its possible, but they usually take the bulk of them from directly below the chin.
assuming your density is greater below your jawline as compared to your cheek, and is like most beards I've seen, I'd guess anywhere around 500