I'm pretty sure it's supposed to be 26 episodes for season one... but most US shows these days have mid-season breaks that make it look like two separate seasons. Which is really annoying for the good shows, especially when it now gives them the chance to have two cliffhanger episodes each season for viewers to sweat over for a couple months.

I have a feeling that it has been announced somewhere, but now I can't find anything to say how many episodes have been officially announced. I guess we wait and see... but 13 played in quick succession seems very unusual, especially for a first season that is meant to sell a new toyline.