V for Vendetta (the film) - What is the explanation for number 5? Why is it important?

It is important to point out the connections between V and the Roman numeral V. For starters, the name of the book itself is five syllables. V is an individual with a vendetta. Each chapter in the book has a "V" in the title. Also in Chapter 1, when the fireworks go off and the letter "V" emerges, it is at once his name and a remembrance of his time spent at the Larkhill Resettlement Camp in Room V.

Every time V leaves a crime scene, he leaves his mark, which is the circle with the letter "V" in the middle. This serves as a reminder too, to the people he seeks for his vendetta the significance of "V." They do not make the connection however.

In Book 1, Chapter 5 Evey asks V the meaning of "V.V.V.V.V." There are five V's that in Latin are "Vi veri veniversum vivus vici" which translates to "By the power of truth, I, while living, have conquered the universe." This gives the reader a sense of foreshadowing as he continues plucking out the government leaders. In Book 1, Chapter 8, Eric Finch makes the connection between Beethoven's Fifth and the opening bars, which he realizes is Morse code for the letter "V."

Most importantly the connection between V and the number V is most strongly made during his incarceration at Larkhill. He was confined to Room V and this is where he made his transformation from, who he was before into a new person. It was the defining point in his life. His confinement altered his identity, so he adopted this as his new profile. He lived the remainder of his life as a product of his time spent there. Certainly his vendetta was a product of that time as well.