This is a very good thing that it only is 12 episodes. There aren't many novels out and the manga is literally nowhere. Best to just do the first novel animated and give it time to get plenty more novels out for later.
Considering that it is easily one of the best shows of the season, it is somewhat likely to get another season or two later. I suppose less likely than Spice and Wolf, but still likely.
Keeping it 12 episodes prevented much filler and random blah, since even the filler ep 6 was really an interesting take on the characters. The series has been concise and will continue to be as long as it stays on point, which is one of its strengths.