Your question is quite broad, it really depends on your needs. Let's say that you have a data structure holding your level information. If the level is the same for every player, one instance is enough. But if you're saying that different people might see certain stuff, you might want to have an instance for every player, plus a global instance that holds all recent data. If a player sees more, his own instance is updated with the global one.
If you implement the multiple instances, be sure to read/write to the correct one at the correct time. To give an example: in Company of Heroes, path finding is (unfortunately) done on the global instance. This means that I can use this path finding to find out if a certain route is available. (let's say my map shows a bridge in the fog of war. When I give a move order to my troops, I can see if the bridge is available or broken by the path my troops will go, even before they can see the actual area).
Technically speaking, the AI should have the same resources as the humans. However, the fair method makes programming the AI a lot harder: you need to implement certain behaviours (such as "smart scouting"). While if you just give them the full map to see, they know where the resources are without scouting. Because creating a worthy AI is a very hard challenge, AI's are often have access to the global map.