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

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