#Limit the blood, not the ghouls#
Do not put a hard restriction on the amount of ghouls, put one on the amount of blood your characters may get. When blood is limited, your characters will have to think twice before spending it.
Play with the rules of hunting, make it very time-consuming, dangerous etc. Make hunting an important part of play if your characters do it a lot, make it narrated (sessions dedicated to hunting may give very interesting results), make it dangerous. What about your noble Ventrue being attacked by a local gang? By the way, are your characters even allowed to hunt where they are trying?
Your problem character is Ventrue, so hunting is already quiet hard for him -- he needs to find an appropriate vessel. Even if he has Herd background, it's size isn't very big, 60 at best if you check V20 book. If he abuses his herd, it will just die out quickly.
PCs may buy blood, but it should be kinda expensive for them. When they buy too much, local blood bank may just refuse to sell more -- some should actually be used on healing...
Non-Ventrue PCs may buy animal blood from butchers, which isn't as hard, but endangers the Masquerade.
###What goes to the armies of ghouls...###
My life as GM would become easier if there was some mechanical support for why vampires doesn't keep armies of ghouls.
They do. The Tzimisce make huge armies of Szlachta, fleshcrafted warghouls. Anarchs have used armies of ghouls because they are, indeed, less expensive than vampires, requiring less blood to be spent nightly (still high, though, if you need to upkeep a huge army). As I remember, ghoul armies were also more or less common during the Dark Ages -- even without Disciplines, the ability to soak lethal damage is a lot.
You may find more info in the Ghouls&Revenants book.