54

54 = 2 x 3³.

54 is the smallest number that can be written as a sum of three squares in three ways: 54 = 1² + 2² + 7² = 2² + 5² + 5² = 3² + 3² + 6².

54 is the number of integer partitions of 19 into distinct parts.

A polygonal number can be represented as nested points arranged in the form of a regular polygon. 54 is a nonadecagonal (19-gonal) number: 1, 19, 54, 106, 175, . . .

54 is 110110 in base 2 and 66 in base 8.

54 is the number of colored squares on a Rubik's cube.

Sources: Wikipedia; Number Gossip