The thing is, there were Forsaken necromancers in Classic. And not only that, but the Forsaken were looking actively for more. This was because, if I recall correctly, they were able to break the grip of the Lich king on other enslaved undead. I think it was also stated there were not a lot, hence the need for more. One of the main examples is Helcular.
So from here we can assume Forsaken see Necromancy fine at least as long it helps them free other undead. Nothing is stated about raising new people (or maybe it is and I don’t remember, it’s been a long time).
Later in cata the Val’kyr join the Forsaken ranks, and we see them raising new undead. The portrayal of this in the quests was a bit odd, since most of them were fighting for the Forsaken just after being killed and raised by them, which doesn’t look quite good with the idea of free will. To reassure the claims of “mind control” many people had, a developer stated this was becasue after being raised there were this state of “frenzy” were the undead would attack anyone who they were directed at. The Forsaken took advantage of this state and used it against their enemies. Later, the new undead would calm down and they would be offered to join the Forsaken, go back to the grave or mind their own busisness.
So here we see the Forsaken are not only cool with raising new people, but also with using them in a weak state of mind. But as far as we see, the Forsaken don’t enslave mindless minions. We also see an apothecary sentenced to true death after making mindless minions from Forsaken forces, leaving to the asumption their limit was in the mind control part. Maybe because of this the necromancy were left to just the Val’kyrs and a few selected, or maybe it was because only the strongest ones could create not mindless undead.
Now… well. We don’t see any limit. Sylvanas raises mindless skeletons in the Battle for Undercity. We see geist being part of Forsaken forces (I think in Darkshore, I’m not sure where I saw them). We see in Before the Storm Forsaken live in a dictatorship were they cannnot have anything that remembers them about Lordaeron (retconning part of Cataclysm quests). And we see Sylvanas torturing Derek’s mind to the point there’s almost no difference to slavery and going against the last and small piece of “Free will is the cornerstone of Forsaken society”.
So, after a review of “how the Forsaken saw the necromancy” timeline, I will answer you.
It always have made sense for the Forsaken to have and train necromancers, but at least in Classic, and with some small logic jumps and assumptions in Cataclysm, there could be an explanation of why they weren’t too many.
Now in BfA, I cannot find any reason for Sylvanas to not have a full force of Necromancers prepared to jump to any warfront to start rasing the Horde enemies.
So the real answer in the end is the typical one: Bad writing