Eleven

Eleven is the natural number following 10 and preceding 12. It is the first repdigit. In English, it is the smallest positive integer requiring three syllables and the largest prime number with a single-morpheme name.

Eleven

is maximal number of digits of National identification number.

Except Switzerland:

As of 2008, an anonymous thirteen-digit number is being issued to all Swiss residents. It is of the form 756.XXXX.XXXX.XY, where 756 is the ISO 3166-1 code for Switzerland, XXXX.XXXX.X is a random number and Y is an EAN-13 check digit.^[45]