In proposing a new edition of Thomas Jefferson's papers in 1943, Julian P. Boyd estimated that Thomas Jefferson had written 18,624 letters in his lifetime.[1] 
Ever since then people have been rounding up or down from that estimate. Despite its curious precision, there is no reason to regard it as anything but the loosest of estimates. For the period prior to Jefferson's acquisition of copying machines and his creation of the epistolary record in which he logged most of his incoming and outgoing letters from 1783 to 1826, one would need a time machine to come up with anything but a wild guess. There is also plenty of room for disagreement over the definition of "letter": Jefferson did not record brief letters of invitation or one-line grocery orders to local merchants in his epistolary record, but he addressed them and sent them, and one could certainly describe them as letters. They seldom survive and are, again, hard to quantify. When the Papers of Thomas Jefferson are complete, we will be able to undertake a solid count of letters printed, noted, and no longer extant but known to have existed. For the reasons stated above, this count will still supply only a lower bound. That lower bound may well be somewhere near the ballpark figure established by Boyd. Until then, it is probably reasonable to use a figure of roughly 19,000 letters written by Thomas Jefferson in his lifetime, if only because it is idle speculation to try to get more precise at this point.
- J. Jefferson Looney, 3/24/2008
