There are 12 in a dozen.
There are 12 different pentominoes.
122=144 and, reversing all digits, 212= 441
There are 12 tones in the modern 12-tone musical scale.
A dodecahedron, one of the 5 Platonic solids, has 12 faces.
There are 12 inches in a foot, 12 pence in a shilling in older money systems.