MySQL: Limit number of rows returned with a given column value

My table has a list of activities, each associated with a customer and each with a date when the activity happened. I want to group by the customer ID and the month and the year, then sort by the month, the year and the count to generate a list of time activity generators by month. SELECT original_actor_id, MONTH(activities.created_at) as month, YEAR(activities.created_at) as year, COUNT(*) as count FROM activities activities WHERE activity_type = 'product_video_completed' GROUP BY original_actor_id, MONTH(activities.created_at), YEAR(activities.created_at) ORDER BY YEAR(activities.created_at) DESC, MONTH(activities.created_at) DESC, count DESC However, I am only interested in the top 5 counts for each month/year combo. What is the best way to limit this to the top 5 higest counts for each month/year combo?