You can group by the intervals and select the counts.
SELECT
count(date_action) as count,
CASE
WHEN minute(date_action) BETWEEN 0 and 14 THEN '00'
WHEN minute(date_action) BETWEEN 15 and 29 THEN '15'
WHEN minute(date_action) BETWEEN 30 and 44 THEN '30'
WHEN minute(date_action) BETWEEN 45 and 59 THEN '45'
END AS intervals
FROM
clients
WHERE
HOUR(date_action) = 09
AND DAY(date_action) = 15
AND MONTH(date_action) = 07
AND YEAR(date_action) = 2013
AND rep_id = 28
GROUP BY intervals