You can place up to 2500 assets in the media library to use at any one time and save it as a project for future use. If you have more than 2500 songs or music composition then you can remove some to add others to use in your future projects. Not sure of your questions. You can try this and let us know if it works in PD18 as I don’t have 2500 pieces of music to try yet.

If you load up just your music clips into the media library, you can right-click on a blank area and use Export Library to save it for later.

Note that exporting the library won't affect where the actual clips are stored, you're just keeping a copy of the library showing them all together in one place no matter where they actually reside on your system. In future pojects, you can use Import Library to bring them all back in.