[RPG] Can a vampire ghoul a carnivorous plant

There is no complete, canon answer.

At least so far as I have found, no source provides an average number of ghouls per vampire or per city. It is not mentioned in Ghouls & Revenants, and if there were to be an official answer, I would expect it to be mentioned in that book. More than that, as Baskakov_Dmitriy mentioned in a comment, there are several variables that would affect it city to city. The most significant factor would be the Prince (at least in Camarilla controlled cities), as creating a ghoul is a technical violation of the masquerade and thus requires the Prince’s consent (so long as the Prince is powerful and respected enough to enforce it…). This means that there would be high variability from city to city and situation to situation. In statistical terms, this means that even if we had an exact mean, it would have a high standard deviation.

Furthermore, on a meta level, the needs of the plot and the type of game being played might affect the number of ghouls that actually appear in a meaningful way even if the storyteller declares that the world itself is officially sticking to whatever the average may be. In an “on-the-run from powerful enemies” type games where the main players move often, ghouls might appear very rarely. In a more stationary, politically focused games, ghouls will probably appear a fair bit, though mostly as background characters. But in a “Ghouls” game as laid out in “Ghouls and Revenants”, Ghouls are likely to be extremely common as most PCs and their peers will be Ghouls.

Slightly more than 1 per vampire is probably a good estimate

This essentially is a guess, but I think it is a fairly reasonable guess. Ghouls are so useful for handling business during the day, providing protection during the day, and providing cover for the masquerade that most vampires will want at least one. (Other non-ghoul retainers can also provide most of those benefits of course, but between the power of the blood bond, the enhancements to the ghouls’ abilities, and the lack of need to protect the masquerade from the ghouls, ghouls generally make better retainers). On the other hand, the law of diminishing returns hits rapidly for a resource such as ghouls. In other words, having one ghoul is a huge benefit, having two ghouls is only slightly better than one, having three ghouls is only very slightly better than having two, and with all the maintenance involved having four ghouls might be more trouble than it’s worth for most vampires.

If all of those assumptions are correct, then I would conclude that most vampires will have one ghoul. Not all will of course. Some will not have the clout to get the permission from the Prince. Some will not have a desire for a ghoul, especially among the Gangrel. But the sizable minority that do not have a ghoul are likely to be roughly balanced by the number of vampires that maintain more than one ghoul.

Altogether, I think that on average, a city will have just slightly more ghouls than it does vampires.

Just to emphasize though, that is both an educated guess and subject to wide variability even in settings where it is true in general. Sabbat held cities will hold fewer ghouls as Ghouls & Revenants states that the Sabbat make less use of the Ghouls. In times war, Ghouls may well be produced en mass to fight for their masters. Etc.

The rules as written all suggest that a ghoul could extract no more than the vitae a vampire had. As mentioned by Sardathrion's answer, vitae measures potency, not volume. Even if there was still physical blood in the vampire, a ghoul extract no more potency from it after the vitae was all extracted. On the other side, a vampire of low generation may hold far more vitae than a mortal could hold the equivalent amount of blood.

As LegendaryDude points out in a comment below this answer:

Another way of saying it is that if you take the same vampire (physical size and stature) and lower his generation (however you might, e.g., diablerie), the volume of blood remains the same (because his size and stature haven't changed). However, his blood has become much more potent. Blood pool on the character sheet, then, is not a measurement of actual blood, but of remaining "blood power."

Incidentally, while this question is not directly answered in plain terms, the concept of ghouls capturing vampires and using them as sources of vitae is discussed in "Ghould and Revenants" for V20. One of the bigger complications with it is the blood bond, unless there is something in place that would handle preventing that.