In my vampire campaign I want to limit the amount of ghouls my players keep, but without setting an arbitrary cap. Ideally I would like there to be some strategic trade-off in the question to ghoul or not to ghoul.
I would prefer if there was a noticeable but not crippling cost to keeping ghouls. You do need to feed them blood occasionally, but not nearly often enough that it becomes a burden. This is far outweighed by the blood you can drink from them when needed (even if usually saved for last resort).
One of my PC:s who is Ventrue and especially protective of his blood pool has the habit of drinking a blood point from his ghouls before feeding them one, practically sidestepping the already small cost completely. I don't like this and have thought about house-ruling it out, but as far as I understand it is fine according to RAW.
I am an experienced GM, but still learning the Vampire rules. So far I have mainly used story elements to balance ghouls. For example you can easily create a ghoul, but a good one is hard to find. A ghoul can also be a vulnerability since it knows your haven location etc.
My life as GM would become easier if there was some mechanical support for why vampires doesn't keep armies of ghouls. Did I overlook something I could use this way?
Edit: Thanks for several good answers that helped me understand I cannot solve this using game mechanics. I will instead play up the vulnerabilities of ghouls even more, for which you gave me some great ideas.