I don't have the exact number, but I can tell you that I frequently have women in my office who are in this situation, where they have to make this decision about whether or not....
Even though there is an exception for violence, it's ineffective because they still have to apply for the exceptions. They would have to notify an immigration official that they are leaving the relationship and are applying for the exception. Many would rather, as one person said, do their time in the abusive relationship and not risk losing their status.