You can group all of your realeasedates by month and year to get a count like this:
SELECT MONTH(releasedate) AS month, YEAR(releasedate) as year, count(r_id) AS number
FROM my_table
GROUP BY YEAR(releasedate), MONTH(releasedate)
ORDER BY YEAR(releasedate), MONTH(releasedate)
This'll give you something like this:
+--------+--------+--------+
| month | year | number |
+--------+--------+--------+
| 1 | 2013 | 13 |
| 2 | 2013 | 8 |
Then you could select the maximum like this:
SELECT MONTH(releasedate) AS month, YEAR(releasedate) as year, count(r_id) AS number
FROM my_table
GROUP BY YEAR(releasedate), MONTH(releasedate)
ORDER BY count(r_id)
LIMIT 1
Which'll give you:
+--------+--------+--------+
| month | year | number |
+--------+--------+--------+
| 4 | 2013 | 19 |
+--------+--------+--------+
Which'll represent the highest month