Rule 714.3a

As a Saga without the read ahead ability enters the battlefield, its controller puts a lore counter on it. As a Saga with the read ahead ability enters the battlefield, its controller chooses a number from one to that Saga’s final chapter number. That Saga enters the battlefield with the chosen number of lore counters on it. (See rule 702.155, “Read Ahead.”)

Find this number in the current Rules