translating this SQL query from a different dialect

I have the following query which I would need to be able to use in
PostgreSQL. This basically limits the number of allowed rows in a
table to 8.

  CREATE TRIGGER <TRIGGERNAME> ON [<TABLENAME>]
  FOR INSERT AS
  BEGIN DECLARE @<VARIABLENAME1> INT
  SELECT <VARIABLENAME1> = COUNT (*) FROM <TABLENAME>
  IF ( @<VARIABLENAME1>) > 8
  BEGIN RAISERROR ('<ERROR MESSAGE>', 16, 1)
  ROLLBACK TRANSACTION
  RETURN
  END
  END

I've tried various combinations but none of them seem to work... any
help to convert this to PostgreSQL would be highly appreciated..

Thank you.