Things that come in groups of 12 are called dozens. The word "dozen" is derived from "douzaine," the French word for "exactly 12." This French word, in turn, is derived from the Latin word for twelve, "duodecim."
A dozen has long been a convenient measuring number. Unlike the number 10, 12 is evenly divisible by most single-digit numbers: 1, 2, 3, 4 and 6. Thus, it is easy to sell items by the dozen or by a fraction of a dozen. The custom of counting in dozens or multiples of dozens probably originated in Mesopotamia. Their sexagesimal system, using 60 as a base number, used the mathematical advantages of 12 and added the 5 and 10 as divisors.
A dozen dozens, or 144, is termed a "gross." It is also of French origin, specifically "gross douzaine" or "large dozen." A "great gross" is a dozen gross. A "small gross" is 10 dozen.
A baker's dozen, or 13, has a somewhat different origin. Medieval bakers often sold rolls and other small baked goods in dozens. However, they risked fines if the items sold did not meet a statutory weight. Therefore, they would often put an extra roll into a batch of a dozen to ensure it weighed enough. A baker's dozen may also be called a "long dozen."