In Dressrosa Luffy enters the Corrida Colosseum Tournament as the 556th contestant “Lucy”, registered with the ‘0556’ number on the back of his shirt.
556 is the number of the chapter where Luffy has arrived to Marineford in order to save his brother Ace .. and here he tries to save Ace’s Fruit.
Also there’s another reason as to why Oda chose this particular number. Luffy was born on May 05 and 5-6 means go-mu in Japanese.
ODA is really a genius!