How much is in a baker's dozen?

A baker's dozen is not 12, it is 13. There is a story for the origin of this practice that dates back to medieval times and bakers throwing in an extra roll or loaf of bread, to avoid being found guilty of cheating their customers based on prices vs quantity of wheat flour sold. In modern times, the practice is more about generosity of your baker; though you might not find a baker who actually gives you 13 when you pay for a dozen donuts.