I know that Life of Pi is based on true events. However, how much of the novel is fact and how much is fiction? The author's notes lend credibility...

It's probably impossible to tell, without author explanation. Certainly, events like the shipwreck are most likely fiction.  In fact, the author "sprinkles the novel with italicized memories of the "real" Pi Patel and wonders in his author's note whether fiction is "the selective transforming of reality, the twisting of it to bring out its essence."

Ultimately, it may not matter which is which, as the author is successful in explaining his tale.