My N9 stats are

9.5GB - for User data
4.2Gb - for Installed apps
2.1GB - for Application data
In total there is 15.79GB available

I got 500 photos and over 900 songs on the phone with 4GB user data free. If you plan to have music, convert it to .mp4a with 144Hz compression and you won't see a loss in quality but will save LOADS of space.

Guide how to convert to .mp4a
Software download for .mp4a

Hope this helps