Maybe you can try it with a Formatstring like this:
$if($grtr($right(%discnumber%,1),1),$num(%discnumber%,1)-%track%. %artist% - %title%,%track%. %artist% - %title%)
This depends on how exactly your current DISCNUMBER are filled.
Only 1 or 2 or 1/5 and 2/5 and so on?
If the right 1 character of your DISCNUMBER content is greater than 1 then the first part with $num(%discnumber%,1)- will be used.
If the right 1 character of your DISCNUMBER content is equal or less than 1, then the second part - after the comma (only with %track%. %artist% - %title%) - will be used.