13, cause that's how many inches my dick is
Hehe, no, really, people tend to pick either 7 because it's the old fairytale number (seven dwarves etc), 13 because it's for bad luck and people like to take chances, or 21 because it's for luck.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram