So why are we saying that the narration is "limited" in a movie where we can see everything that's going on with all the jurors? Well, we might have access to the different juror's opinions, but these jurors also talk about tons of stuff that we know nothing about.
Usually, narration is limited to a character's mind. But in this case, it's limited to a spatial location. We hear all kinds of stuff about witnesses and testimony that's happened before this movie begins, but all we really know about it is what we hear secondhand from the jurors. In this sense, the movie offers us limited info and then gives it to us like breadcrumbs to a bird.