27.

27.
Cube of 3.
Atomic number of cobalt.
Number of amendments to the U.S. Constitution (so far).
Width of the tennis court (in feet, without the galleries).
Brian Jones, Jimi Hendrix, Janis Joplin and Jim Morrison: all died at the age of 27.
The Italian Job: 27 million dollars in gold.

But most importantly, 27=1/2 of 54.