iTunes won't transfer all my songs to iPhone 12.

First of all, you need to confirm that the songs that don't make it to the phone actually play in your iTunes library.

If they do, then check to see if your Sync options are set to synchronise your entire library, or have you set it to sync only selected playlists etc?

• if the option is set to synchronise only selected playlists, artists, albums and genres, then the missing songs need to be in one of the selected items or you need to select additional items (such as a genre)
• if the option is already set to sync entire library, check to see if the missing songs are ticked (checked) in the library (the Songs view is the easiest view to show this). If your iTunes is set to sync only ticked songs and videos, then any unticked song (checked song) will not be added to the phone