CREATE TABLE table_12159115_3 (series__number VARCHAR2, written_by VARCHAR2)

How many episodes were written only by William N. Fordes?
SELECT COUNT(series__number) FROM table_12159115_3 WHERE written_by = "William N. Fordes"
SELECT COUNT("series__number") FROM "table_12159115_3" WHERE "William N. Fordes" = "written_by"