Community

See number of available slots for entity registration

I am using entity registration to allow visitors to signup for workshops. 12 workshops are listed on a general overview page and I would like to indicate the number of available/open slots for each workshop.

The overview page: http://www.hdmp.com/program-healthoneday
An example of a workshop: http://www.hdmp.com/node/282

Could anyone tell me how to do this? Do I have to query it myself? How do I display the value?