¿Cómo contar las ocurrencias en columnas separadas por comas?

Puedes usar:

SqlFiddleDemo

SELECT sub.val AS tags, COUNT(*) AS `count`
FROM
(
  SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(t.tags, ',', n.n), ',', -1) AS val
  FROM tab t 
  CROSS JOIN 
  (
     SELECT a.N + b.N * 10 + 1 n
       FROM 
      (SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) a
      ,(SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) b
      ORDER BY n
  ) n
  WHERE n.n <= 1 + (LENGTH(t.tags) - LENGTH(REPLACE(t.tags, ',', '')))
) sub
GROUP BY sub.val