Limit items added to calendar

Hi, I would like to know the best way to go about limiting the number of items that can be added to a calendar in a SP2010 site. I was thinking of an event receiver which would look at the entries in the list and only allow 2 entries per day. Is this difficult to do?

I may need to have different categories of entry onto the one calendar so perhaps I need to have additional filtering. 

Is there an easy way to generate popup message to the user if no bookings are available? Is it possible that this will work for lists filled in by infopath forms?

Thanks.