I wonder--could you tell my jumbled story in exactly one hundred chapters, not one more, not one less?
The number one hundred is a very "ordered" number. Ending with the very even and very finite number provides a perfect and formal conclusion to the story. Pi feels quite strongly that he do this because he did not leave Richard Parker with that same suitable conclusion.
It's important in life to conclude things properly. Only then can you let go. Otherwise you are left with words you should have said but never did, and your heart is heavy with remorse.
Pi is convinced that he must "conclude" his story "properly" because of his failure to do so with Richard Parker. Pi further laments that he never said "thank you." He never celebrates with Richard Parker over their dual survival. Before Pi knew it, Richard Parker is gone. Pi feels only remorse about not seizing the day.
The ultimate irony is that although the book ends with a very finite number, the story runs "full circle." It begins with Pi in Canada. It ends with Pi in Canada. "I'll tell you, that's one thing I hate about my nickname, the way that number runs on forever." Yet another reason why Pi wants to end the book on the note of conclusive success.
As an end note, I should mention that many scholars believe the 100 chapters of the novel correspond to the 100 years in the 20th century. For example, in 1929 the stock market crashed and people began to struggle for simple existence. In chapter 29, Pi talks about people moving for the "hope of a better life" and because of "the wear and tear of anxiety." In 1969, the USA sent a rocket to the moon. In chapter 69, Pi sends rocket flares into the sky to signal ships.
There is no doubt that the number 100 is vastly significant in Life of Pi.