It has to cover up to vol 6 because each season covers a story arc. The first season covers the first story arc which consists of vol 1-2 and some side stories from vol 8. The second season will most likely only cover the second arc which consists of vol 3-6.
If that's the case the pacing for the second season will be faster in which one volume will be covered in 3 episodes on average, whereas in the first season one volume was covered in 5 episodes.
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