When exercising the idea of being mindful and focusing on the details and intricacies of the movement, muscular contraction/effort, and intention eludes most people. If you want to succeed and achieve desirable results from your training it’s not good enough to just show up and move weight around with no thought or mind to how you are lifting, and what you are lifting with.
This video demonstrates how to move your spine through flexion and extension with minimal stress to the musculature. This can be useful place to start in the progression to strengthen the spinal musculature, especially for those who have back pain and a low tolerance to force with exercise.
Exercise execution is one of the most, if not the most, important component for achieving desirable outcomes with training. Before concerning yourself with how many reps you should perform you first need to master your execution.