Thanks to Jovan for the heads up.
That is not a fixed number, as I hear the pickup is for a minimum of 13 episodes, with a likelihood for more. The move was made to give NBC brass scheduling flexibility. Of the renewed dramas, Jennifer Lopez’s Shades of Blue is a 13-episode order; that is how the show has been set up, as are most heavily serialized starring vehicles on TV, including The Following, How To Get Away with Murder and Aquarius. But the rest are shows that have been doing full seasons: the three Chicago series (the newest, rookie Chicago Med, produced 18 episodes because of a late launch), Law & Order: SVU, The Blacklist, Blindspot and Grimm.
Its final Season 6 episode count will depend on when it debuts and how many episodes it can run in a row.