It has been shown that one stretch of 15 to 30 seconds is signigicantly better than no stretching, two is even better, three is still better, but four stretches does not seem to significantly improve over three. When stretching for improved ROM/flexibility, always stretch after at least 10 minutes of moderate/vigorous activity.
There is a lot more to understanding stretching. I teach a series of CEC classes on this subject and more. Check out my website www.hawaiifitnessacademy.com for more information.