Think about it. Multiple hands would be handy around the lab. He can take on non human shapes.  Why doesn't he have multiple hands?

Reed isn't a shape shifter. He's just super elastic. Spawning an extra pair of hands from his back  probably goes quite a bit beyond his ability.

he can stretch all of his limbs and entire body but he can't create new limbs thats beyond his ability but he could stretch so excess skin on his back to make simple hands but not fully formed limbs

Just super elastic, not a shapeshifter like plastic man