I just want to make sure I am getting this right, but a Create Greater Demi Plane that is augmented with a casting of Create Demi Plane to make it Bountiful says "The demiplane provides enough plant-based food (nuts, grains, fruit, fungi, and so on) to support one Medium creature for every 10-foot cube of the demiplane." but it does not say for how long. Is that once, and then you have eaten it all up until another growth cycle occurs, or per year, or per day?
China has one of the most efficient farming systems in the world, able to feed 20 people per acre for a year. A 20th level casting of Create Greater Demi Plane creates a space about 0.9 acres (400 10' cubes). Even if the Bountiful trait is SUPER bountiful it should not be able to produce enough to feed 400 people a day - that's 146,000 people a year!
Am I missing something in this math, or is the idea of the spell being able to feed 400 people per day (forever, with Permanency) really what it is saying. It could be read as meaning 400 people per year or season, there is no given time frame.
It says it supports one Medium creature per ten-foot cube. There doesn't need to be a duration specified, because support is an ongoing thing. So long as the plane is active, it will produce enough produce for that many people. Is it much of a surprise that an 8th/9th level spell cast by a 20th level caster can feed a lot of people?
Your math is misleading. The plane does not feed 146,000 people per year, it feeds 400 people for a year, just like an acre of Chinese farmland feeds 20 people for a year. It turns out that the most powerful magic of a 20th-level caster is 20 times better at feeding people than some low-level commoners with an acre of land.
My players are looking at making a demi-plane with the intent of harvesting it every day. You are right, it is 400 people per day, per month or per year -- which is still pretty damn awesome -- but it is in scope. Thanks! My mind is rested.
Glad to help out!