There is no official line. Three sum up the novel: "one cannot live before a mirror," in Book VII; "I am this tree. Tree, cloud; tomorrow a book or wind," in Book VIII; and the one that gives the book its title, in Book I, in which Moscarda was one hundred thousand, all with the same name, inside a body that was one and none.
The first is Moscarda's, said to Anna Rosa. She looks at herself in every mirror; he says that life, in order to see itself, must stop and pose, and that to know oneself is to die. When a feeling sees itself, it freezes. From this comes the sentence, which sums up the mirror of the whole novel, spoken by a narrator who has spent the book examining himself.
The second is from the close. In the last chapter, already at the asylum, after saying that the name is an epitaph and that life does not conclude, he says that he is this tree, the cloud, and that tomorrow he will be the book he reads or the wind he drinks: all outside, wandering. It is the sentence in which the book comes closest to an answer, and the text leaves open whether it is liberation or dissolution.
The third is the one of the title: the moment when the three words come together for the first time, in Chapter IV of Book I. The hundred thousand Moscardas he was, not only for others but also for himself, all within a body that was one and none, alas.