Plex is unfortunately completely album-centric. There’s two ways to make single tracks work:
- put each individual song in its own folder and give it an album name, (name of the single, or an album it was on) (see @OttoKerner 's helpful suggestion above)
- put them together in a folder and tag them all with the same album name (eg, “Various Singles”), with track numbers and with album artist “Various Artists”
The first option is the most work, and will clutter your library with loads of 1-song “albums”, but things like search will work. The second option basically creates a new compilation album, which looks tidier in the albums list, but comes with the downside of Plex’s problematic handling of compilations (for example, broken search, tracks won’t get played in Radio).