There's no such thing as a "magic number." As the commenter above said: 1200 calories is the minimum that a person should eat (actually, the minimum that a female should eat; I believe that 1500 is the minimum for men.)
But everyone is different and every body has different needs. Some people can thrive and do very well on 1200 calories a day; for others, 1200 calories isn't close to what they need to get through the day.
Personally, I can't do 1200 calories. I mean...I can *net* 1200, I suppose, but I exercise an hour a day, six days a week, and there's just no way that I can survive on 1200 calories a day. On the days I work out, I usually eat around 1,700 calories and burn off approximately 500 calories through exercise. It still leaves me with 1200 *net* calories, but at least this way I'm not passing out all over the place (the way I can be if I try to just eat 1200 calories.)
The key? Everyone's different. Everyone loses weight differently. What one person can survive on is not what everyone can.