L numbers are for plecs which are yet to be scientifically named. Common bristlenoses have been scientifically named (Ancistrus sp.) and so do not have an L number.
For reference, Planetcatfish.com would be the place to look.
expanding on the above, it depends what type of bristlenose you have, if it is common then exactly as idlefingers states no L number for ancistrus sp3.
picture may help although ancistrus can be hard to pin down without a catch location.