If you're going to start reading All-New X-Men with the addition of X-23 in issue 20, you may want to start with issue 19. It picks up X-23's story after Avengers Arena by maybe a day or less.
X-23 features in it as bald from the radiation exposure over in Avengers Arena. Potentially with minimal to no memory or traumatized from the events that took place.
There are a few questions though.
Did Laura run away from where she was recovering? (She does hate labs and hospitals, but was it instinctual this time?)
Why hasn't her hair already started to grow back? (Radiation might still be wearing off. More on this soon as KP dives into X-23's healing factor!)
How come Kitty had no issues with the adamantium in Laura's claws? (The density of the metal gives Kitty Pryde difficulty with phasing.)
How did the Purifiers find Laura?
Why hasn't Wolverine, or Gambit, or anyone of the Jean Grey School noticed she ran away?
Be sure to pick this issue up before issue 20 where she formally joins the team! Many of these questions and more could be answered!