It's a surprisingly high percentage... Here's an article about it, apparently the choice to have a child or not apparently plays pretty heavily into how you feel about it. I think it's cruel to judge people for regretting having a child, they already deal with the guilt of that as well as raising the child. There are all sorts of feels and viewpoints towards the world so I think as long as the parent still does their best it's best to respect their feelings rather than judge them as a bad person for having them.
The only percentage that I can find in that article that lists 'unwanted' is 10, and the study is anything but scientific.
For most people birth control is about when they will have their children, not whether or not they want children. Having kids is - in mosts parts of the world - still the norm and not having kids the exception. Maybe that will change over time, it is now socially acceptable not to want children whereas say 200 years ago that meant that you were either destined to become part of some religious organization as a monk, nun or in the clergy.
If I ever do meet such a person I will gleefully judge them as a bad person.