From the Breaking Bad Wikipedia article:
AMC announced on May 16, 2008 that Breaking Bad will be renewed for a second season:
“From critical praise to strong ratings and devoted audience, Breaking Bad further reinforced AMC as a top producer of high-quality, distinctive television”, said Charlie Collier, Executive Vice President and General Manager of AMC. “Breaking Bad is a powerful, intelligent thought-provoking series that clearly resonates with viewers and critics alike. We’re exited for a second season of Breaking Bad with a new provocative storylines that will delve deeper into the next chapter of the life of Walter White”. The second season, unlike the first which ran a brief seven episodes due to the writer’s strike, will consist of thirteen episodes.