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.