The number twelve (12)

Number Theory:
* Even composite number
* Abundant number
* Highly composite number (the smallest)
* Harshad number
* The number of edges in a regular icosahedron
* The atomic number of magnesium

Mathematics:
* The number of months in a year
* The number of hours on an analog clock
* The number of signs in the zodiac
* The number of imperial gallons in a British barrel of beer
* The base of the dozenal numeral system

Religion and Mythology:
* The number of apostles of Jesus Christ
* The number of tribes of Israel
* The number of disciples of Buddha
* The number of labours of Hercules

Culture:
* The number of letters in the English word "dozen"
* The number of pennies in a shilling (pre-decimal Britain)
* The number of rounds in a boxing match
* The number of notes in the chromatic scale
* The number of jurors in a criminal trial (in some jurisdictions)

Sports:
* The number of yards in a football field
* The number of wickets in a cricket innings
* The number of players on a basketball team
* The number of holes in a regulation golf course
* The number of innings in a baseball game (regular season)

Other:
* The atomic number of carbon
* The year 12 AD
* The twelfth letter of the modern English alphabet
* The number of ounces in a pound