Lydia, stretching can be one of the most beneficial ways for natural back pain relief. This is one of the most crucial things you do all day to help maintain flexibility in your body.
You can stretch as many times as you need a day, just as long as you are maintaining a light to medium intensity and holding the stretches for 40-60 seconds.
There is a point of “overstretching” but that usually happens when you stretch for longer than 20 minutes in one setting or when you stretch too intensely.
One of the best things I can recommend is to stretch when you begin to feel tightness in a particular area of the body. For example, if you are sitting for an hour and your lower back and legs start to feel tight, immediately start stretching your hamstrings, gluts, quads and calves. Don’t wait for the perfect time to stretch, learn to do simple stretches wherever you are.
I hope this helps! Martin.