How common are predictive dreams?

More common than you might think. Almost everyone at some point in their life will have one. Most often it is something mundane, like seeing an old friend or having their favourite meal for dinner. Those don't usually make a big impression on the person so they don't really remember. It is the "big" or unusual predictive dreams that make an impression. If you dream of a car accident and then are in one, or dream of a flood and the river over flows its banks, you tend to notice. If you think you are experiencing precognitive dreams, my best suggestion is to start a dream journal. When you wake up write down the dreams in as much detail as possible, and date them. Then if something similar happens, record that in as much detail as possible with the date it happened. One or two close incidences might be of interest, but several very accurate dreams which prove out could well mean you are experiencing precognitive dreams. Then you will need to decide what, if anything you intend to do with them.