Triggers fire once for each manipulation of the table regardless of the number of rows affected. If the one row is affected, the trigger will fire. If five rows are affected, the trigger will fire. If zero (0) rows were affected, the trigger will fire. One of the biggest mistakes developers make is creating triggers that don't support multiple records.

>IF (SELECT somefield FROM inserted)=0
>INSERT INTO sometable ect....

If this code exists in a trigger that fires for a multi-row operation, it will generate an error. Subqueries of this type must return a single value in a single row. To produce the same functionality in a multi-row friendly way, you would need something like this:

INSERT INTO sometable ...
SELECT ...
FROM othertable INNER JOIN inserted
ON inserted.column = othertable.column
WHERE inserted.somefield = 0

-Mike