Disable Webform Componotent (field) after certain number of submissions
I would approach this by using 

hook_webform_submission_insert (which is triggered after the submission is inserted into the database) 

http://www.drupalcontrib.org/api/drupal/contributions%21webform%21webform.api.php/function/hook_webform_submission_insert/7

Once in that hook i would use webform_get_submission_count to get the number of submissions , if this was the last allowed submission i would then unpublish the node and create a redirect to a "this class is now closed page". 

http://www.drupalcontrib.org/api/drupal/contributions%21webform%21includes%21webform.submissions.inc/function/webform_get_submission_count/7

You could also approach it by using hook_webform_submission_presave or a custom form validator to generate a form error.