The original title of Machiavelli's Discorsi was Discorsi sopra la prima deca di Tito Livio, literally (according to Wikipedia) "Discourses on the First Decade of Titus Livy". "Decade" here refers to the first ten books of Livy's Ab urbe condita. However, the tenth book ends with the end of the war against the Samnites in 292 BC., while the Discorsi also discuss events and topics from later books. So why does the title specifically mention the first ten books?
This might be a remnant for Machiavelli's original goal of writing a running commentary of the first ten books in Livy's A Urbe Condita. Several other passages in the book may also be remnants of this original goal, namely those where Machiavelli quotes a sentence from Livy at the start of a chapter, e.g. at the start of Book 2, chapter 2, and Book 2, chapter 23.
Since Machiavelli was mainly interested in the republic as a form of government, it is not clear why he would focus only on Livy's first ten books, since these books cover Roman history up to 292 BC, while the end of the republic is covered much later (books 117-133). The final form of the Discourses on Livy is not simply a commentary on Livy's work, even though that is Machiavelli's most important source.