You can register one copyright or you can register a collection of copyrights (like an album of songs). If the copyrights are unpublished, you can register up to 10 titles on one application.
When creating an application for a collection of copyrights, the authors/owners must all be identical for each copyright. The roles each author played and the publication information must be identical across all copyrights too. For example, if 9 out 10 songs on an album have the same authors and owners, they can be filed on one application. If the 10th songs has a different combination of authors and owners than the other 9, it must be filed separately.