I'd guess to about 20 or so letters a month on average.  My current boss is a great one for email but it does depend so much on the person.  Boss before last was a great letter writer - probably a 100 or so a month.  But as times are changing, more and more goes via email.  As Gee said, if it needs a hard copy, original signature, letter is the only way, even using pdf wouldn't be acceptable.