Note that this switch is automatically assumed (with a defaut value of 1, i.e. the first song) if you are adding a header to the file, even without explicitly giving the switch.
Example: psidTool.EXE -s5 /S3 "Mazing SoundTrack.SID"
Example: psidTool.EXE "Mazing SoundTrack.C64" -a *.SID /i$1000 -p4099 /title:"Mazing - The Game" -S3 /s5