How many years were recorded when world ranking was 21st?
CREATE TABLE table_23449 (
    "Index (Year)" text,
    "Author / Editor / Source" text,
    "Year of publication" text,
    "Countries sampled" real,
    "World Ranking (1)" text,
    "Ranking L.A. (2)" text
)
SELECT COUNT("Index (Year)") FROM table_23449 WHERE "World Ranking (1)" = '21st'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: How many years were recorded when world ranking was 21st? ### Input: CREATE TABLE table_23449 (
    "Index (Year)" text,
    "Author / Editor / Source" text,
    "Year of publication" text,
    "Countries sampled" real,
    "World Ranking (1)" text,
    "Ranking L.A. (2)" text
) ### Response: SELECT COUNT("Index (Year)") FROM table_23449 WHERE "World Ranking (1)" = '21st'