You have a few options on how to handle something like this. You could handle it on the server side with a model scope to check each individual day. However, that could be more taxing on the server if a calendar had several events; not to mention that you're calling the scope about 30 times per calendar view. An alternative would be to leverage client side calculations for the number of events. However, you could still run into issues with the client side taking a while to calculate the number of events.

Would your case use be for a smaller summary calendar? I know that the current version of FullCalendar does have a scaling feature where it will gracefully hide events and display a number of events for that day.