How many audio tracks can I add per slide?

Articulate Presenter allows you to add one audio file per slide, whether through Record Narration or Import Audio.

Using one audio file over multiple slides
If you have one large audio file you would like to add for consecutive slides you can do so by following the steps outlined here:
Split Audio Across Slides

Importing multiple audio files
If you have multiple audio files you would like to import you can do so by following the steps outlined here:
Import Multiple Audio Files

Background Music
If you would like to add background music in Presenter 5 you would need to create a Playlist in Articulate -> Library and Options which you can add to your slides using the Slide Properties Manager.