Only 3 people know. My parents and a friend I had who I opened up about it because she also self-harmed.
My mom was extremely freaked out. Under lots of stress and I could see her shaking and heard about her being unable to sleep. She tried to be understanding but failed. She checked my body every week after that. My dad was the opposite. He's always been less emotional. He said it hurt him a bit but said that it's just a "stupid thing" that I will look back on in the future and see that it wasn't "that serious". It wasn't just a few scratches though, I had been self-harming for about a year and drawing deep cuts, which I still have scars of years later. Sadly he's always thought about my mental health like this. Even my suicide attempt he called a stupid act.
Other than that, people have seen the scars and questioned it, but I shrug it off or make up some excuse.