This question has come up a number of times on MLR. While we would all like a quick answer to tap into the riches of the music library world, the fact is there is no easy answer. There are hundreds of music libraries that serve different markets, from royalty free sites, mid level boutique, to high end mega libraries. What may work for one composer will not necessarily work for another. There are tales of composers signed to high end mega libraries making big bucks and others signed to the same library where their music just sits, generating little if any income.
The real truth is that this is a long game business that requires patience and persistence. You will also need a lot of material. The number that gets bandied around a lot is 1500 tracks spread out over a number of different libraries both exclusive and non-exclusive. In time you will build up a catalog that should generate a nice income stream. There will always be those that catch the brass ring but, as a general rule, allow yourself at least five years.
by Art Munson