My 6 month old daughter has begun rolling over so much in her sleep that she whacks her head against the wooden railings during the night. What can I do to stop her from hurting herself in her sleep?

Most babies will be rolling by 4 - 5mths of age & many become very active & will roll into the sides of the cot, or get arms or legs through the slats of the cot.
Many babies will also prefer to sleep on their side or tummy.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Does she wake when she rolls into the side of the cot?
This is quite common & there does not appear to be any strong evidence that baby can cause an injury by doing this.
If you have specific concerns/signs of injury then please be in contact with your GP or Maternal/Child/Family Health Nurse.