It is probably one disc but chapters that heavily focus on this and that aspect.

Chapter 1 - Midgar
Chapter 2 - Chasing Sephiroth
Chapter 3 - The Black Materia/Aerith's Death
Chapter 4 - The Knowlespole
Chapter 5 - Great Materia/Cloud's Confabulation
Chapter 6 - Shinra's End
Chapter 7 - Birth of a God