Guitar Hero: Metallica features 28 Metallica master recordings. These tracks span the band’s career and form the Metallica-focused core of the 49-song on-disc soundtrack.
The remaining 21 songs are master recordings by other artists chosen as Metallica favorites and influences. That split is why answers such as 49 and 21 can seem plausible: 49 is the total number of master recordings, while 21 is the non-Metallica portion.
The game launched in 2009 as the second Guitar Hero title centered on one band, following Guitar Hero: Aerosmith. Finding the original recordings was reportedly difficult for some early Metallica material, including songs from Kill ’Em All.
The 28 figure refers specifically to Metallica’s songs on the game disc, not later downloadable content or the number of songs importable into other Guitar Hero games.