11

11 is the integer succeeding 10 & preceding 12

properties
11 is the smallest prime p such that 2^p-1 is composite. 2^11-1 is 2047 = 23*89

11 is the largest known n such that 2^n contains all even digits. 2^11 = 2048 (no others up to 2^1000000)

11 is a repunit prime

11 is the largest known multiplicative persistence

An 11 sided polygon is called a hendecagon or undecagon