As many as your application needs.
Here is another testing program. It instantiates several cards, using a different variable to refer to each as it is constructed.
public class CardTester
{
public static void main ( String[] args )
{
Holiday card01 = new Holiday("Santa");
card01.greeting();
Holiday card02 = new Holiday("Tinkerbell");
card02.greeting();
Holiday card03 = new Holiday("Elvis");
card03.greeting();
}
}
The program outputs:
Dear Santa, Season's Greetings! Dear Tinkerbell, Season's Greetings! Dear Elvis, Season's Greetings!
Could you write the main() program using only one reference variable?