how many have an area of at least 20 km ?
CREATE TABLE table_204_851 (
    id number,
    "serial number" number,
    "river" text,
    "name" text,
    "area (km2)" number,
    "district" text,
    "co-ordinate" text,
    "altitude (m)" number
)
SELECT COUNT(*) FROM table_204_851 WHERE "area (km2)" >= 20
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 have an area of at least 20 km ? ### Input: CREATE TABLE table_204_851 (
    id number,
    "serial number" number,
    "river" text,
    "name" text,
    "area (km2)" number,
    "district" text,
    "co-ordinate" text,
    "altitude (m)" number
) ### Response: SELECT COUNT(*) FROM table_204_851 WHERE "area (km2)" >= 20