From a recent Cartoon Network press release,

Justice League Unlimited: The Justice League, led by Batman, Superman, Wonder Woman and Green Lantern, has grown much larger because the threats are even greater. This season, Black Canary, Shayera, Vigilante, Vixen and The Question step up to face such villains as Lex Luthor, Felix Faust and Tobias Whale. And Superman takes on Cadmus in a battle that will determine the future of the League. Twenty-three new episodes will air this year.

Those twenty-three episode presumably include the current 13-part season and a new 13-part third season. Cool.