How many items are in a baker's dozen?

Thirteen, and the extra loaf was insurance against a flogging. Medieval English bakers sold bread by weight under laws such as the Assize of Bread and Ale, and selling a loaf that came in light carried real penalties, from fines to the pillory. Weighing dough is easy; predicting how much water a loaf loses in the oven is not, so a batch could fall short through no fault of the baker. Adding a fourteenth loaf to every twelve, or a thirteenth to a dozen sold on, made shortfall almost impossible. The extra piece was sometimes called the vantage loaf, and it went to the middleman who resold the bread, which is how the custom survived as a merchant's habit rather than a legal requirement. The phrase outlived the law and now just means a small bonus.

A long hundred, another medieval counting quirk, meant 120 rather than 100.