Dozen is another word for the number twelve. The dozen may be one of the earliest primitive groupings, perhaps because there are approximately a dozen cycles of the moon or 
months in a cycle of the sun or year. The dozen is convenient because its multiples and divisors are convenient: 12 = 2 × 2 × 3, 3 × 4 = 2 × 6, 60 = 12 × 5, 360 = 12 × 30. The use of twelve as a base number, known as the duodecimal system (also as dozenal), probably originated in Mesopotamia (see also sexagesimal). Twelve dozen (122 = 144, the duodecimal 100) are known as a gross; and twelve gross (123 = 1,728, the duodecimal 1,000) are called a great gross, a term most often used when shipping or buying items in bulk. A great hundred, also known as a small gross, is 120 or ten dozen (a dozen for each finger on both hands). A baker's dozen, also known as a long dozen, is thirteen.
The English word dozen comes from the old form of the French word douzaine, meaning "a group of twelve" This French word is a derivation from the cardinal number douze ("twelve", from Latin duodĕcim) and the collective suffix -aine (from Latin -ēna).
English dozen, French douzaine, German Dutzend, Dutch dozijn and Spanish docena, are also used as indefinite quantifiers to mean "about twelve" or "many" (as in "a dozen times", "dozens of people"). Wikipedia
Then there are other interesting phrases related to dozen:
"Six of one, half-dozen of the other" says that two things which people refer to differently are actually the same thing. A "dozen" is a counting word that represents twelve (12) of some particular item, so a "half-dozen" is equal to six (6) of that item, and saying "six of one" is equal to saying "a half-dozen of the other." The "one" and the "other" refer to the two things which you are saying are not so different. Example: "I say she's a stewardess. She says she's a flight attendant. It's six of one, a half-dozen of the other." Although something has been said in two different ways, they ultimately mean the same thing.
"A dime a dozen"
So cheap you can buy twelve for ten cents. Anything that is plentiful with little value. Possibly this phrase originated in the early 1900's as a sale discount for candies that cost a penny. Easy to come by, next to worthless.
If a thing is very common and easy to get, we say it is "a dime a dozen." Example: "Do you think I should buy this now and bring it with us?" Answer: "Don't bother; those are a dime a dozen where we are going."
There is no need to get excited or worried about finding something that is a dime a dozen. Example: "Look what I found!" Answer: "That's nothing special; those are a dime a dozen."
It is easy to find a dime (a 10 cent US coin), and a dozen (12) of something is a common, everyday unit of measure. You are not in a hurry to get a thing which is a dime a dozen because it is not so special and you could get one any time you wanted. Example: "I don't need friends like him; they are a dime a dozen"
Quotes:
Simplicity, simplicity, simplicity! I say, let your affairs be as two or three, and not a hundred or a thousand instead of a million count half a dozen, and keep your accounts on your thumb-nail.”
Comments