How many Knights feature in Edmund Spenser’s The Faerie Queene?

The correct option is 12.

Edmund Spenser's epic poem The Faerie Queene was originally planned to consist of 12 books, each representing one of the 12 private virtues as defined by Aristotle. Each book was to be centered around a specific knight who personifies a particular virtue (such as the Redcrosse Knight representing Holiness, or Sir Guyon representing Temperance).
Although Spenser only completed six books (along with a portion of a seventh, the Mutabilitie Cantos) before his death, the architectural design and the framing letter to Sir Walter Raleigh establish that there are 12 primary knights representing these 12 moral virtues under the overall frame of Prince Arthur (who embodies Magnificence, the perfection of all virtues).