I've been thinking on the topic too.

Basically, you have to have your stable population, and then they need to produce all these "food" babies on the side.

To have a stable population, each woman needs in average to produce 2 kids, 1 male, 1 female.

Let's say a woman can start safely having babies around 15, then once every 18 to 24 months.

So to have a stable population, a girl can have 2 babies by the age of 18. Then, assuming a 50/50 ratio of girls/boys at birth the rest is bonus.

Let's assume a woman can have babies until 45, that means each woman could produce 18 offspring that could become food.
You could potentially raise this number if you assume that some of the food offspring have babies themselves before being consumed.
Although in our stories, 15 is usually already old for becoming food.

So definitely not enough for them to become a staple food, but also enough that assuming women reproduced as much as safely feasible, it should be enough that families could probably afford human meat once or twice a year.

Now even with that, that leaves us with a problem, which is that if we assume as often in our stories that girl meat is the preferred food, how do we deal with the boy's overpopulation?

You could assume that for some reason (an illness, natural evolution shift, ...), the girl/boy ratio at birth shifts. In which case, a 1/18 boy/girl ratio would be ideal for these numbers.

You could also assume artificial insemination to selectively breed girls.

But that's all a pretty industrial process, even the assumption that a woman will breed nonstop from 15 to 45 kinda borders on slavery or some sort of religious fanaticism. Which I guess could be possible settings for that kind of universe.

Now if you wanted to increase that supply or ease the burden, you could make it so women can give birth younger, for example with a medication. Though the main issue for that is that they're not physically mature for giving birth So you'd need to come up with something for that. surgery, or maybe some kine of intensive training at a young age for "widening" them.

If you assume a girl starts pregnancies at 6, then you add 6 potential offspring, raising that number to 24.

You could also assume women can give birth longer, like increasing lifespan like Edward said. If you assume they can give birth until 100 years old, for example, then you add about 37 offspring. However, with people living longer, that also doesn't really increase the supply per year significantly.

Something helpful would be to ensure most of these pregnancies are multiple pregnancies. If you assume for example that they have triplets in average, then even with normal ages, they can have 60 children each (76 with early pregnancies), 58 of them being potential food. Still not staple food level, but already better.

Now if you want to go to staple food levels, then I guess the only way would be to have some population that doesn't eat meat (or human meat at least) and still reproduces.
That could be a meat farm (which would make the artificial insemination and multiple pregnancies easy to explain), or maybe a class system.


So yeah, to sum things up, to have a setting with a stable population where human meat is consumed more than once, possibly twice a year, you pretty much have to have a livestock population that doesn't consume human meat (probably no meat at all) and is basically dedicated to breeding as much as possible. And this population has to be larger that the cannibals population.

If we wanna math it out:
In most developed people consume about 100kg/person/year of meat. Raising to up to 120 in the US.
The human body weight is about 14% bones, and let's say 1% non edible organs.
That means if we had human as our main meat, we'd have to eat about 115kg of human cattle every year.
If we estimate that a cattle girl is aged 9 or 10 in average, that would put her at around 30kg of weight.
So we'd need to eat about 4 girls a year.
Considering an active breeder can normally produce about 18 offspring before age 45, and assuming they're culled when not productive anymore, a breeder produces an average of 0.4 meat girls per year.
That means realistically, you need 10 breeders per meat consuming human.
Considering there are around 8 billion humans, we'd need around 80 billion breeders, which would be a real struggle to feed.

We can make that a bit better with the multiple pregnancies hypothesis. If they produce 60 children each, that 1.33 meat girl/year for each cattle. So we only need 3 breeders per meat eater (bear in mind that includes those not of breeding age). That's only 24 billion breeders to produce all the meat.
But even then, considering you need to grow the livestock around 10 years before being consumed, 360 billion cattle would need to be kept at all time if you count breeders and meat girls.

That's a ratio of 43 cattle per meat eater (3 breeders, 40 meats).
So if we want a more reasonable hypothesis, let's assume that the total population must be kept at 8 billion.
That means we have 182 million people that are "meat eaters", 546 breeders, and 7280 million meat girls, ranging from babies to 10 years old.

The workload of taking care of that many cattle heads would be pretty intense, but probably manageable. Ranchers can raise about 1000 cattle per worker, so about 1 in 23 people would need to be a rancher.
Possibly, you could unload some of that work onto the breeders, having them trained to take care of their calves.
Which would be kinda necessary, because ultimately, as we mentioned, human meat would be way less efficient than traditional livestock, and so we're essentially trying to do the same workload with 1/44 of the active population contributing.


So as an alternative to a human farm system, we could imagine a universe where the population is highly religious and follows a fertility cult with a class system.
The lower class would essentially be the cattle and the working class.
They would have food restrictions, making them vegetarian, with a food regime that hastens puberty, induces frequent multiple pregnancies, and changes the balance of boys/girls at birth.
Women would have a sacred duty to either sacrifice themselves to their rulers, or serve them by procreating as much as possible.
Various religious ceremonies and rites would guide them to do that from a young age.
Training young girl's holes with ceremonies that require fucking larger and larger dildos.
A coming of age ceremony could involve them getting ritually impregnated.
Regular ceremonies would involve intercourse for all girls in a position to become pregnant.
Although most of the girls would be ritually offered as religious sacrifices to the higher class in an ascension rite, where they would become chosen, to offer themselves to their rulers, fulfilling their ultimate duty in life

The higher class would be the rulers by divine rights, much less numerous, and would feast on the lower class' offspring.
I imagine girls and women in that class would be much more free to dispose of their body, although I can't imagine the religion would be entirely without sexual rites for the higher class either. But probably more centered around ritual orgies. The offered girls would probably also be ritually enjoyed, offering themselves as sexual toys before being turned into food.

Alternatively, the sacrificed girls could be chosen at birth, essentially becoming a third class of meat girls.
The breeders only would have rites training them for breeding and giving birth.
That would leave the meat girls to have a religious regime with foods designed to enhance the quality of their meat and flavor, and rites raising them to sexually serve their higher class masters without deforming their bodies like the breeder's rites might.
Another possible setting could be a pandemic that causes girls to randomly and frequently fall pregnant from a young age without intercourse, essentially cloning themselves, causing an overpopulation of girls that are themselves gonna become pregnant.
The population becoming out of control essentially forcing governments to perform culling and encourage cannibalism.
Though I think the "pandemic" trick is more of a classic and ends up feeling a bit more like a deus ex machina.