Every artist has a different answer to the question of "how many layers?", but knowing the difference between direct and indirect painting is key to forming your own workflow. This is an old concept, but it makes the leap to photoshop beautifully, allowing the artist to work on layers independently from one another. There was a time when layers meant you had to wait for paint to dry -- luckily, the digital process allows you to skip that part.
If you like the idea of "indirect painting" and want to know more, check out the "Grayscale to Color" series in the store.