Great question. The GM can add new Faction Sentences (up to ten) at any time during prep or play! That extra four slots is there to encourage customization even from groups that like to play content as written in the book: there is a lot of prompting power in giving someone four blank lines after giving them six pre-written ones!
in general, I like to start games with just the six, and then fill in others as the campaign continues to try to reflect the action of the story.