There are a total of 50 Letter Scraps in GTA V.
Finding all of them is needed for the "A Mystery, Solved" trophy or achievement. They are also required to reach 100% game completion. The letter scraps can be collected at any time, even in the very beginning of the story. Unlike with some other GTA V collectibles, there are no special requirements or side missions necessary to make them appear in the open world. You can also collect them with different characters if you want, doesn't need to be a specific one.
The letter scraps are small, glowing pieces of paper. Some of them can only be reached with a helicopter! You can easily get one from the South Los Santos hospital helipad, or the Sandy Shores helipad.
After finding all 50 letter scraps you must go to the green "Killer" question mark while playing as Franklin. There you will meet Leonora Johnson's killer (Dreyfuss). You must kill him in order to unlock the trophy or achievement! Only collecting the 50 letters won't be enough, you must finish this side mission!