Voltaire did not systematically keep copies of the letters he sent and seldom kept those he received. In a sense, it was his first posthumous editors who, in assembling 4,500 letters, ‘created’ his correspondence. Later editions continued to add to the number, so that Theodore Besterman’s second, so-called ‘definitive’ edition of the correspondence contains 21,221 letters, of which more than 15,000 are by Voltaire. It remains the reference edition, even though new letters regularly come to light: see the most recent list here.
Voltaire’s correspondence is included in the Electronic Enlightenment database and new discoveries are gradually being added, making it possible to study the letters in new ways. High-quality images of all the Voltaire letters at the New York Public Library are also included.
To refer to Voltaire’s letters, it is standard to use the numbering of the Besterman edition, where each number is preceded by a D (for ‘definitive’), for example D123. In the case of letters discovered since the completion of this edition, there is a protocol for creating an identification number: see Nicholas Cronk, ‘La correspondance de Voltaire: la première mise à jour (2011) de l’édition de Th. Besterman’, Revue Voltaire 11 (2011), p.195-96.