The question about Santa Claus's reindeer does not directly relate to a specific AP US History key concept, as it pertains to a cultural and folkloric tradition rather than a historical event or process.
The number of Santa Claus's reindeer is part of a cultural tradition originating from the 1823 poem "A Visit from St. Nicholas" (also known as "The Night Before Christmas") by Clement Clarke Moore, which is not directly tied to the historical developments or processes covered in AP US History.