Is addison's disease a rare disease?

Yes. Addison's disease is quite rare, thought to affect about 40-60 per million people, or 0.004% to 0.006% of the general population.
Not rare. Addison's disease affects about one in every 20, 000 people. It affects an equal number of men and women, and can affect people of all ages. Addison's disease, also called primary adrenal insufficiency, is much less common than secondary adrenal insufficiency, which is a similar disease that's caused when the pituitary gland fails to send the right stimulating signals (acth) to the adrenal glands.