NBC's release announcing the pickup doesn't say how many episodes that means and only says "a production commitment for the remainder of the 2012-13 season."
 
For the season, using Nielsen's confusing "most current" averages (including some amount of live viewership, some Live+7 and some Live+3), "Chicago Fire" is averaging 8 million viewers and a 2.4 rating among adults 18-49. 
 
Of more note in terms of timing, on Wednesday (November 7) night, "Chicago Fire" became the first new drama on ABC, CBS, NBC or FOX this season to top its premiere results with any subsequent telecast. That's a confusing way of saying that in today's TV biz, shows tend to peak with their premieres, just like movies tend not to do better in their second weekends than their first weekends, unless they add theaters. [In effect, "Chicago Fire" added theaters on Wednesday, or at least it got the advantage of a "The Voice" lead-in, rather than its normally lackluster "Law & Order: Special Victims Unit" lead-in.]
 
"We all love this thrilling new drama with its sexy breakout cast. We are excited to see it gaining traction on our schedule," blurbs NBC Entertainment President Jennifer Salke. "Executive producer Dick Wolf and all the producers have done an amazing job developing a high-stakes series with lots of great heroic characters whose exploits are depicted in Dick’s typically epic storytelling style. We are incredibly invested in this one and can’t wait to hear about their plans for the rest of the season." 
 
"Chicago Fire" was created by Derek Haas and Michael Brandt and the ensemble is led by Jesse Spencer, Taylor Kinney, Eamonn Walker, Monica Raymund, Lauren German and David Eigenberg.