Rules Custom Condition for registration capacity
Looking at your function, I believe you need to rename it to something like mymodule_rules_condition_info. The hook you are trying to implement is hook_rules_condition_info(). Did you find that your custom condition was showing up when you were editing a rule?

function MYMODULE_rules_condition_info() {
   $conditions = array(
     'mymodule_event_is_at_capacity' =&gt; array(
       'label' =&gt; t('Event has reached maximum registrations'),
       'group' =&gt; t('MY MODULE'),
       'parameters' =&gt; array(
         'registration' =&gt; array(
            'type' =&gt; 'registration',
            'label' =&gt; t('The registration event'),
           ),         
         ),
       ),
     );
     return $conditions; 
 }


Semantically you may want to call your condition 'mymodule_event_is_at_capacity' and it should only need one parameter which is the capacity that you will provide in the Rules UI.

The function you will be writing mymodule_event_is_at_capacity($capacity) will query for the number of (active) users in Drupal, and if it is greater than or equal to $capacity, it will return TRUE.

All of this can be kicked off by add a new Rule that reacts on the "After saving a new user account" event.