I had a mastectomy of my left breast with lymph node removal 12 years ago would it be safe for me to get a tattoo in that arm?

Depends : If you have chronic swelling of the arm from the lymph node dissection, you are best using the other arm.

Lymphedema: If you had surgery to remove the lymph nodes under the arm during a mastectomy which is common i would not recommend a tattoo on that arm as if you get an infection it could cause your arm to develop lymphedema which could be debilitating and chronic . Get a tattoo on another area would be my suggestion if you want to do that.