Filename depending on type of album (unique artist vs. various artits)

If am not sure, how to create a script for the following rule:
If album has only one artist file naming like
(artist) (tracknumber) tracktitle
else
(tracknumber) (artist) tracktitle

Seems to be easy, but I am a complete newby in this scripting.
Can anybody help me please?
Thanks in advance
Michael

$if($eq($lenmulti(%artists%),1),%artist% %tracknumber%,%tracknumber% %artist%) %title%

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