Conference registration form/site with limited places available
Session = content_type.

One session = one node of type session

Every session has a field "available spaces"

Create a custom form programatically which you pass the session id in a field of type "value". 
https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#val

In the validate function of the form check if the field "available spaces" for the session with that id is greater than 0, otherwise decrease it and in the submit function store the user id with this session id.

Regards.