What is the number of played when points against 645?
CREATE TABLE table_17920 (
    "Position" real,
    "Club" text,
    "Played" real,
    "Won" real,
    "Drawn" real,
    "Lost" real,
    "Pts For" real,
    "Pts Agst" real,
    "B.P." real,
    "Points" real
)
SELECT COUNT("Played") FROM table_17920 WHERE "Pts Agst" = '645'
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: What is the number of played when points against 645? ### Input: CREATE TABLE table_17920 (
    "Position" real,
    "Club" text,
    "Played" real,
    "Won" real,
    "Drawn" real,
    "Lost" real,
    "Pts For" real,
    "Pts Agst" real,
    "B.P." real,
    "Points" real
) ### Response: SELECT COUNT("Played") FROM table_17920 WHERE "Pts Agst" = '645'