There can be any number of instances of an area running at a time (limited by hardware). Each is independent of the others.
Area Instances are identified by an ID that is assigned when they start (called an area "spin up").
The Area Instance with the ID of 0 is special: this is called the Edit Instance, and is where all world building takes place. The Edit Instance will save all changes to the database and it is this data that becomes the source template which used by all other instances of the Area that spin up later.
All other instances other than the Edit Instances are called Play Instances. No changes to the world in these instances are saved to the database. When the instance shuts down, any changes are discarded.
All Area Instances are independent of each other, but can communicate through HSL remote calls.
Area instances can also be connected so that they will appear to a player as a contiguous space, using the Seamless World feature which is available in version 1.22.