Sure, round figures, lets call it 800K. And I bet the vast majority of them knows. It doesn’t take much of an examination for a doctor to determine location of heart and liver.
In fact many of them don’t, since the body is mostly symmetrical and apart from cutting them open or doing an MRI, you can’t really tell (which isn’t a big deal in most cases, because most medical procedures work regardless of this condition). Also, the heart is located almost in the middle, so there is not much difference.
Listening to the chest with a stethoscope, or your ear will tell the location of the heart. Percussing over the liver, but not finding a solid structure e.g. liver, which sounds different than a hollow structure, e.g. lungs would also help in identifying unexpected organ locations. I’m curious how you came to know that many of them don’t know? Do you keep a register of people with this condition, but don’t tell them?