The last episode of season 1 of Rome was EP 12, so the 1st Ep of Season 2 becomes 13. It really depends on how the individual show decides to count their episodes. Some count them straight, and some do it more with a number denoting the season followed by a number that denotes the episode, which resets with each new season. In Romes case they've decided to continue counting from season 1 with Ep 12 to Season 2 with Ep 13.