I guess it depends on the context, as Instance is quite a generic term. It could refer to the name of a SQL Server instance (e.g. in the string .\SqlExpress, SqlExpress is the instance name).
It could also refer to programming where you create an instance of an object. E.g. in this code:
Code:
public class Person
{
public string FirstName { get; set; }
}
....
Person somebody = new Person() { FirstName = "John" };
the somebody variable refers to an instance of the class Person.
For more information:
http://en.wikipedia.org/wiki/Object_(computer_science)
Since you create as many objects / instances as you need, you can't really say how many instances there are in ASP.NET.
Hope this helps,
Imar