How much music theory does a songwriter need to know? You might as well ask how many brushes a painter needs or how many instruments should be in a band. How much theory you'll need is up to you. If you're a classical composer, you'll probably want a college-level education in theory and composition. If you're a film, TV, or jingle writer, it really helps to know how to orchestrate and write out parts. If you're a songwriter who hopes to hear his or her songs on the radio someday, the amount of theory you need to know can be determined by how many options you'd like to have, how many styles you'd like to explore, and how much money you'd like to make.

Did the part about the money get your attention? Like any other advantage you could name, an education in theory doesn't guarantee success, but it can give you a distinct advantage in certain situations — it can help you to write better songs, more songs, and more kinds of songs. That can mean a lot more money in the long run. Do you need to know music theory to be a successful songwriter? The answer is no. You don't need it in precisely the same way you don't need a vehicle to get from New York to Los Angeles. You can get there by walking, but a car, train, or plane is a faster, easier, and more certain way of getting there.