CREATE TABLE table_204_851 (id NUMBER, "serial number" NUMBER, "river" CLOB, "name" CLOB, "area  ( km2 ) " NUMBER, "district" CLOB, "co-ordinate" CLOB, "altitude  ( m ) " NUMBER)

how many have an area of at least 20 km ?
SELECT COUNT(*) FROM table_204_851 WHERE "area (km2)" >= 20
SELECT COUNT(*) FROM "table_204_851" WHERE "area (km2)" >= 20