A good question and here are some websites that provide some answers to the question...

http://www.teenhelp.com/teen-health/cutting-stats-treatment.html

http://depression.about.com/b/2008/02/05/1-in-6-teens-self-harm.htm

http://www.tree.com/health/teen-behavior-self-harm.aspx

As these websites say, it is difficult to get a sense of what the true percentage is because of the lack of reporting that is done on these issues.  It is also sometimes difficult to identify the many different forms of self harm that teenagers enage in.  

When teens cut, it is always good to try and provide them with a replacement behavior that can take the place of cutting.  Teens cut for a reason and the reasons they cut are vast, it may be that...

-They are emotionally numb and cutting allows them to feel something.
-They are experiencing emotional pain and cutting provides a release.
-It is a way for them to feel control if they feel they have no other control in their life.
-They are addicted because cutting can be habit forming.

Whatever the reason, try to identify a behavior that they can engage in that will give them the same result as cutting.  It may be exercise, journaling, art projects, listening to music, or any number of other things that can provide them with that same emotional payoff.  

Good question and if you need additional help, call our hotline!

Counselor, Dominic
Boys Town National Hotline-A hotline for parents and teens
1-800-448-3000
www.parenting.org