Baldur's Gate 3 sticks to the classic three-act structure but refers to these as Chapters, so there are three Chapters in the game (and a tutorial Prologue). The winning combination of setup, conflict, and resolution is itself a tale as old as time, but developer Larian Studios' own interpretation is slightly misshapen.
Just like in Larian's last game, Divinity: Original Sin 2, most of Baldur's Gate's massive runtime will take place in Chapter 2. That's when players will reach the legendary city itself, where most of the game's content is.