Hello,
The amount of data usage would be the size of the files you stream or download to your device. Other things that count towards you data are email (including attachments), web browsing, YouTube, Apple Music, Spotify, and other music streaming, app purchases, apps that use Internet connectivity, etc.
If they are songs purchased from the iTunes Store, then each song would average around 3-15 MB each. Most movies from the iTunes Store are around 1-3 GB.
Hope that helps,
C