Peter has more than twelve, I'm sure. Think of it this way: if he had less fears than Tris, the presumed second-best in Dauntless history (after Tobias) why would he be ranked fifth? (Was he ranked fifth? Or fourth?!)
As an informed guess, I'd say injustice as he sort of admits speaking to Four "What if somebody has seven fears and another has twenty? That's not exactly fair." Which can also exemplify being overshadowed, but driving both home is when he stabs Edward in the eye for ranking above him. In which in his own twisted way was justified.