How many books did Edmund Spenser originally plan for The Faerie Queene?

Edmund Spenser originally conceived of his epic poem, The Faerie Queene, as a religious, moral, and political allegory consisting of 12 books. Each book was intended to recount the adventures of a knight representing a specific moral virtue.123

Although Spenser outlined this 12-book structure in an introductory letter included with the first edition of the poem in 1590, he ultimately completed only six books and a fragment known as the "Mutabilitie Cantos" before his death.23