Conference registration form/site with limited places available I have to build a little conference-site with a registration-form. 

The conference is 1 day and has different sessions in the morning and different sessions in the afternoon. Those sessions have limited spaces available. 

Users should be able to register for the conference by the registration-form and if there are no more spaces available for a specific session, they should get a message that this is no longer available, so that they have to choose another session.

What is the best solution for this, knowing that my client wants 1 registration form for the entire day. Otherwise I was considering entity_registration. With the "1 registration form" in mind, I was considering to use webform and webform_validation or webform_rules, but there is no standard solution for the limited spaces available.

Is there a good solution for this problem?

Can someone help me?
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.