WITH RECURSIVE
-- таблица исходных данных
test AS (SELECT '16' val UNION ALL
         SELECT '20 440' UNION ALL
         SELECT '21' UNION ALL
         SELECT '16 440 21'),
-- опорная таблица чисел
cte AS (SELECT 1 num
        UNION ALL
        SELECT num + 1
          FROM cte
          WHERE num < (SELECT 1+MAX(LENGTH(val)-LENGTH(REPLACE(val, ' ', '')))
                       FROM test))
-- основной запрос
SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(val, ' ', num), ' ', -1) element, COUNT(*) count
FROM test, cte
WHERE LENGTH(val)-LENGTH(REPLACE(val, ' ', '')) >= num-1
GROUP BY element

SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(val, ' ', num), ' ', -1) element, COUNT(*) count
FROM test, ( SELECT 1 num UNION ALL
             SELECT 2 UNION ALL
             SELECT 3 UNION ALL
             -- ...
             SELECT 8 UNION ALL
             SELECT 9 UNION ALL
             SELECT 10 ) cte -- допустим, не более 10 элементов на запись
WHERE LENGTH(val)-LENGTH(REPLACE(val, ' ', '')) >= num-1
GROUP BY element