There are 12 2-cent stamps in a dozen. This is a kind of riddle. A dozen of any object is always 12, regardless of what the object is. In this case, it is not important what the value of the stamp is, a dozen of them always equals 12.
There are many variations to this riddle, such as, "Which weighs more, a pound of lead or a pound of feathers?" Though an object made of lead would generally weigh more than a feather, a pound of each would be equal because they both weigh exactly 1 pound. In both of these riddles, the key is to focus on the unit (a dozen, a pound) and not the extra information about the object.