>>281506297
The only reason we think this is 12 episodes (aside from most anime only get 12 episodes) is because they have only put 3 BD volumes in its website that cover up to episode 12.
So either they will butcher Volume 3 and Volume 4 by giving each only 2 episodes (which is unlikely, but still possible), or this is actually 24 episodes adaptation and they are going to announce second set of BD (3 volumes, covering 12 additional episodes) when it reach (or about to reach) episode 12.