How often do failed relationships cause one to self mutilate?

A failed relationship is one of the top reasons for suicidal ideation and suicidal completion without a mental health diagnosis. It's a sort of acute crisis, like losing a job, public humiliation, etc. that can be very demanding on a person even when they've never experienced mental health symptoms otherwise. 

That would be NEVER.

At least not for people who are healthy mentally and emotionally.  Because people who are healthy may be sad at a breakup but they will move on and keep functioning well.

However, people who are mentally and emotionally ill to begin with will take a relationship loss, a family death, getting fired or losing a job to the extreme and act out in inappropriate ways.

The rest of us just take life as it comes.

You'll find these articles very useful.

https://www.jw.org/en/library/magazines/g201507/br...

https://www.jw.org/finder?docid=102006046&prefer=l...

Freedom from Self-Harm: Overcoming Self-Injury with Skills from DBT and Other Treatments  - this book has been endorsed by Marsha Linehan, who created DBT, the best treatment for borderline personality disorder, which has serious self-injury as a frequent symptom. If you go to the Amazon ad, you'll see that readers GREATLY appreciate this book. One said, "This book helped me tremendously."

Advice about a variety of mental health problems, including a book recommended by professionals for breakups - How to Survive the Loss of a Love, in my answers and you can click and read if you like.

Most people have the ability to cope with the stress of a break up.  They keep themselves busy and work out their frustrations and feelings, if tat doesn’t work they speak with a good friend or family member about what happened.

Sometimes relationships end and leave one partner or the other are left devastated and can’t sort things out on their own. can feel foolish, alone, isolated, they may feel angry or some vindictive need to get back and get even with the former partner.  Anger, shame, sadness, fear and just about every emotion in between may be running through their minds even being so overwhelmed that suicide and self harm are the only ideas that come to mind.

When you reach your frustration point, seeking professional help may be the only safe route left.  If you don’t have money or insurance there is councils going out there for someone to help, al, you need tell them is you are or have been feeling suicidal and they should get you in to see someone ASAP.

Wow, that's naivé. The only thing that "causes" self-harm or suicidal tendencies is one's own psychological dysfunction and inability to handle emotional stress. See a therapist.

hopefully they dont cause that

I have to say that this trend to self-mutilate is a new thing. I never heard of people "cutting" until a few years ago. Now I understand that cutting causes the release of endorphins, which make people feel better. But I have to tell you that in previous generation, after a breakup, women ate gallons of ice cream and cried with their girlfriends, while men went out with friends and got drunk- both healthier behaviors than self-mutilating.