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
thebody 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
thatselect 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,
butI 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