CREATE TABLE table_204_929 (id DECIMAL, "location" TEXT, "name of mill and\ grid reference" TEXT, "type" TEXT, "maps" TEXT, "first mention\ or built" TEXT, "last mention\ or demise" DECIMAL)

how many mills were built or first mentioned after 1800 ?
SELECT COUNT(*) FROM table_204_929 WHERE "first mention\nor built" > 1800
SELECT COUNT(*) FROM "table_204_929" WHERE "first mention\nor built" > 1800