On a 12MP camera how much memory does each picture take?

It depends. Usually the camera stores the image as a JPEG which has lossy compression. So your image size can vary based on the compression. For example, a picture with mostly blue sky may compress more than a picture with more colors. In the raw mode, you should retain the 12 MB per picture. Anything else will most likely be less.

An example: a 4,000 x 3,000 picture is made up of 12,000,000 individual pixels -- each capturing a color (assume 8 bit) which equates to 12MB. I'd guess on average each picuture using JPG compression would result in a ~3MB file.