I don't see
Monster being adapted into an open-ended ongoing series.
I looked back over a few series for HBO. The one closest to
Monster might be
Band of Brothers. That show was adapted from a book by historian Stephen Ambrose and follows a single Airborne company from basic training to the end of World War II. It was complete in ten episodes.
Many of HBO's recent original offerings like
Girls or
The Newsroom started with six-episode runs with renewals depending on audience. Season one of
The Sopranos began with thirteen.
At 24 minutes per episode, the 74 episodes of
Monster work out to 29.6 hours. No television executive would commit to a show of that length. Del Toro and company will be lucky to get thirteen 56-minute episodes; that would give them 12.1 hours to tell the story.