FOOD: (1) Dietary Requirements
People have to eat. No amount of magic can change it.
Why does this matter?
This is the most basic limit your population has: if they don’t get enough food, some would just die off. Produce it, import it, or conjure it, but your people have to eat enough food.
Agriculture requires peasants, peasants need cultivable land, arable land makes beautiful landscapes… and you suddenly discover that an orc country filled with endless plains of ash is simply impossible (you’d need at least some hospitable green lands somewhere out of the view).
How much food people need?
A short answer is 1,800 cal per person per day. A long one…
Average healthy 21–25 year old adult males with active lifestyle need around 3,000 cal per day; females, as they are usually smaller, need only 2,400 cal. Thus sayeth the Center for Nutritional Policy and Promotion of USA. Younger or older people need slightly less nutrition, but can be more sensitive to malnoursihment. The historical data is consistent with these benchmarks, but starvation was still quite common.
For the quick-and-dirty estimates, we will turn to some Wiki quotes of FAO data about 1,800 cal being the average minimum daily energy intake; this number is pretty believable but the link is long dead. Going a thousand years into the past probably would not change it that much: while nearly everybody worked physically enough to be considered sportsperson in our gentler age, there were a lot of youngsters that ate much less than adults.
How different this is in your world?
To be fair, I don’t know. Probably, not that much.
First, you can tweak the social and economic structure. The number above assumes the society of early XXI century, not the one suitable for contemporaries of Richard Lionheart. In some societies, minimum nutrition requirement can be slightly lessened if the society is very young, poor, or war-torn: children eat much less than adults (and families had a lot of children in Medieval world), females eat less than males (and there would be much fewer men if the state is warring constantly), peasants can find some minor alternative sorts of nutrition like wild plants or roots, or to be constantly undernourished without dying immediately. If, on the other hand, your town (or the whole civilisation) is full of heavily training troops, you may need to increase dietary requirements up to 3500 cal for soldier camps.
Second is the fantasy itself: non-human races, magic or deities may change everything too. Burly orcs and dwarfs probably need to eat more than ordinary humans: 3000 cal for a civilian may be okay. Tiny goblins or halflings would not need as much, and probably would be able to suffice on measly 1200 cal. Giants… let’s not even start on the dietary requirements for half-ton humanoids!
The food requirements may also be lowered or heightened by the magic or theological intervention, but it is really questionable. For these factors to be important, you need a high magic setting: blessings or extensive modifications should cover a significant portion of the population. If nearly every village has a priest, daily blessings can lessen the dietary requirement of the peasants during the more work-intensive times like ploughing or harvest and be somewhat important. Timely supersurgery or magical influence from adepts of Life magic or fleshcrafters (Vicissitude is an example) can be important part of the soldiers training, whether the spell rebuilds the body in a way that decreases minimum nutrition or has an unfortunate byproduct of increasing it (in w40k, space marines transformation actually does both: they are far larger than humans and have augmented digestion). All in all, active influence from mages or priest seems questionable: one-time magical intervention can be dangerous, and constantly reinforced effects can be draining for the caster.
Generally, the racial structure should really be accounted for and the magical/theological influence may be disregarded unless you create a really magical setting: the mages or priests will be preoccupied with something fancier than tending to peasants.
In my future calculations, I will use the very same 1800 calorie benchmark for future articles. If you come up with a different energy intake given your unique race, just increase or decrease it accordingly.
