https://www.gamespot.com/articles/super-smash-bros-ultimate-soundtrack-has-over-800-/1100-6461025/

Super Smash Bros. Ultimate for Nintendo Switch will have more than 800 music tracks for players to listen to while they smack about some of the most iconic video game characters of all time. Details on the game were revealed in the latest Nintendo Direct, during which the My Music feature was discussed. According to Nintendo, Ultimate will let users select specific tracks for each stage and then decide how often music tracks play on that stage.

While that function has been in previous games, the way music is categorized in Super Smash Bros. Ultimate is being changed. Until now, music tracks were tied to stages; however, now tracks will be organized by series instead, so you can listen to any track from the series your battlefield is associated with.