Where did you hear about the five episodes? But it sounds about right though. Joy didn't really go into too much detail when she did that interview at the Zooey event but I do remember her saying that she would be there "for a little while" which I took to mean to be about a handful of episodes. I think Charlotte Rampling's character, Dr. Evelyn Vogel, is the only one that will be on for most of the season. But Dexter's seasons are only about 12 episodes so even if she's on for 5, that's like half the season.
I'm really happy she's trying out different tv shows, and different genres at that, but I think it would kind of nice for her to maybe even try for a small, independent movie. The last movie I think she did was BIOA and it was...not good to say the least lol. I don't know, like maybe a cute rom com?