How many farmers does it take to support a non-farmer? In modern times a single farmer can feed 100 people with his crops. In medieval Europe, nine farmers can feed themselves plus another person. Mediterranian climates, without their harsh winters, allowed for more generous food production.
The question is important because the number of people involved in food production determines how many people are doing anything but farming. Ever wonder why science, culture, art, and most other things barely advanced for millenia and then suddenly exploded? It wasn't because people suddenly stopped being stupid; it was because they were doing something besides trying to produce enough food to survive.
The game assumes 3 people can produce enough food to feed 5. That's fairly generous, and assumes good crops, a mild climate, solid work and food animals, and bountiful land that is well-irrigated. It also assumes excellent plows and a good crop rotation system that doesn't leave land fallow for extended periods of time.
However, this is fantasy. Magic often fills a void left by technology. Maybe the gods graced this land with abundant potential for growth. Maybe everyone has access to life magic that improves crop yeilds. Maybe high-energy, nutritious plants and ideal herd animals exist that means one person can feed three, or five. After all, given all the horrors this world has to offer, can't it have some good in it as well.
If you want to go this route, the rules allow for a variant where one person produces enough to feed 5. This allows you to focus your population on growing in other areas. It's basically an easy mode that allows you to do more with fewer people.