Well, I think it's pretty obvious that the first "season" was really two seasons released at once - two 13-episode chunks that each form a very separate arc with a different primary antagonist and subplots, and even a short timeskip between them. They even called it "Part 1" on Netflix, rather than "Season 1." The fact that they've confirmed Anton Yelchin already did the voice work for this season before his death indicates the show was always meant to be 39 episodes (three seasons) long. Dreamworks usually releases their Netflix shows in blocks of 13 episodes; why they didn't with Trollhuters, I dunno.
Edited by Chris McFeely, 10 February 2017 - 01:42 PM.