PostgreSQL Function: how to know the number of the returned results of the Query

Hi List;

I am building a function in the SQL (PostgreSQL), so I will be able to call this function using a SELECT query. Inside the body of this function, I was need to do the following but not able to know how:

I have a SELECT statement (inside the function script itself), and I need to know the number of the returned rows of that select query, if it is one row or 2 or 3 , ... How? In which paramter I will be able to check this?

For example, I have the following function:

CREATE OR REPLACE FUNCTION get_credit_time(text, text)
RETURNS integer AS
$BODY$
DECLARE
BEGIN
rate numberic(9,4);
SELECT rate from voiptariff where id= 9;
IF num_rows ==1 THEN -- As example, but I am asking how to do it?
.............
ELSE
.............
END IF
END
$BODY$
LANGUAGE 'sql' IMMUTABLE
COST 100;
ALTER FUNCTION get_bool(text) OWNER TO gkradius;

In this function, I need to check the number of returned rows of the statement: SELECT rate from voiptariff where id= 9; because based on it I am going to build if statment, How?

Any help?

Regards
Bilal