How common are real phobias?

Very. There are many possible phobias and they are all real. Common are fear of small spaces and spiders and snakes. All are real, but not usually disabling.
Ask others and see. It is best to recognize and confront your your phobia. Ask others and you will see how many people around you have exaggerated fears of things others don't even notice. Fear is a real emotions. Exaggerated fear-- phobia--is also real and common. If you or a loved one has phobia, do not need to live with it. It is treatable.