Supply and demand (IN)

It's a bit confusing. It actually lists the supply and demand modifiers for every province in the world. It should actually read as: If province X produces Grain it will need 50% less fish, if it has a port it will need 10% more fish etc. etc. If you counquer a grain producing province you will not do anything to supply and demand simply because supply and demand work on global scale.

There are ways to influence supply and demand, but it's not easy . For example Grain provinces have a -90% supply penalty if there is a military unit in them. Also each province that is part of a country which is involved in a war will demand 10% (not sure, working on memmory) more Grain. Start with Russia in 1700. Put a military unit in every one of your grain provinces to reduce supply and declare war on the 5 biggest countries in the world. You will see the price of Grain increasing

A good example of late game goods is Ivory. Check out Tripolly - they have Ivory producing provinces. Notice that the of Ivory atm is very low - probably between 3 and 5. Now check out the demand modifiers - there are a couple of +50% increases that require Market, Constable etc. When the world advances enough and starts building those improvements the price of Ivory skyrockets, because each province that has one or more of those improvements demands 50% or more Ivory -> higher global demand-> higher price.

Yes and no. The number of provinces is indeed static, but the supply and demand is NOT hardcoded. Everyday some province somewhere changes in a way. For example it gets looted - supply from that province drops by 90%. A market is build someplace (the Ivory) - demand increases a bit. Etc. etc.

Also consider the New World, You get random resources from each province. Some will be grain

I fou like numbers, see the wiki : http://www.paradoxian.org/eu3wiki/Trade_goods
The factors apply to every single province in the world.