The amount of songs that can fit in a gigabyte depends on the size of the songs.
A typical 3 minute MP3 song is about 3MB. So a gigabyte could hold around 300 songs.
A higher quality song, such as a WAV file, can be much larger. A 3 minute WAV file could be as large as 50MB. So a gigabyte could hold about 20 WAV files.