According to The Independent article, 'The Letters of Ernest Hemingway, Volume 2, 1923-25, review' there are "6,000 or so of Ernest Hemingway's surviving letters, addressed to more than 1,900 recipients and running to nearly three million words" which would eventually fill at least 17 volumes.
Here are samples from three of those letters:
To F. Scott Fitzgerald: “I wonder what your idea of heaven would be— beautiful vacuum filled with wealthy monogamists, all powerful and members of the best families all drinking themselves to death. And hell would probably [be] an ugly vacuum full of poor polygamists unable to obtain booze or with chronic stomach disorders that they called secret sorrows.”
To a friend – on bullfighting: "It isn't just brutal like they always told us. It’s a great tragedy—and the most beautiful thing I’ve ever seen and takes more guts and skill and guts again than anything possibly could. It’s just like having a ringside seat at the war with nothing going to happen to you.”
To a friend – on learning to write: “God knows you’re in the most depressing and discouraging surroundings–but that’s what makes a writer. You have to catch hell. You’ve got to take punishment … Write a lot–but see a lot more. Keep your ears and eyes going and try all the time to get your conversations right.”