The entire Divine Comedy is made up of 100 cantos. Its three parts—Inferno, Purgatorio, and Paradiso—contain 34, 33, and 33 cantos respectively, creating a carefully balanced total.
The extra canto belongs to Inferno and serves as an opening canto for the whole poem. If that introductory canto is treated as a prologue, the structure becomes one opening canto plus 33 cantos for each of the three major realms. This arrangement reflects Dante’s strong use of symbolic numbers, especially three, which evokes the Christian Trinity, and its multiples.
The number 100 also suggests completeness. Each cantica is built from cantos, which are further written in interlocking three-line stanzas called terza rima. The poem contains roughly 14,233 lines, though counts can vary slightly by edition and textual conventions. Readers sometimes mistakenly say that each section has exactly 33 cantos; that is nearly true, but Inferno’s opening canto is why the complete work has 100 rather than 99.