Here is a deep feature analysis of why the 12-episode count was the secret weapon of Dexter Season 2.
The length forced the audience to sit in the cabin with Doakes. We felt his hunger, his rage, and his confusion. By Episode 12, when Doakes finally meets his end (via Lila's interference), the audience has been put through a wringer of moral ambiguity that only a 12-episode timeline could manufacture.
This episode marks the moment Dexter stops running and starts hunting his own blackmailers (James Doakes). In a shorter season, this pivot would happen around the midpoint (Episode 4), leaving little room for the psychological fallout. Because the season was 12 episodes long, the writers had the luxury of spending the first half of the season breaking Dexter down—his panic attacks, his increasing sloppiness, his isolation—before allowing him to rebuild into the "monster" he embraces in the finale.
The main cast returned for Season 2, with a few significant additions that drove the plot: Dexter: Season 2 (2007) - Cast & Crew - TMDB