This is because Spotify used to have a limit of 10,000 songs in a user’s library, but not anymore. The company has announced that they will be killing off the 10,000 song limit in libraries, meaning that in theory, you should be able to keep an unlimited number of songs in your library at one go.
According to the company, “This makes it possible to save songs, and albums on Spotify to your heart’s content. Avid Spotify users like me will no longer have to curate their Library, or remove songs before adding new tracks. This also avoids that anxious feeling that appears when you’re deciding whether to save full albums, or select single tracks in anticipation of the ‘epic collection my friend’ pop up.”
However, we should note that this only applies to your general library. As far as playlists are concerned, there is still a limit on the number of songs which is set at 10,000. It will also not apply to offline downloads, where if you’re a Spotify Premium subscriber, you can only download up to 10,000 songs on five different devices for offline listening.