Shows how to set up a Participants Database signup form that limits the number of signups

pdb-signup-limit.php

<?php
/**
 * 
 * signup form that limits the number of signups
 *
 */

// set the maximum number of signups
$limit = 100;

// set up the query to get the current total
$query = 'SELECT COUNT(*) FROM ' . Participants_Db::$participants_table;

// get the total
global $wpdb;
$total = $wpdb->get_var( $query );

 ?>
<div class="wrap <?php echo $this->wrap_class ?>" >
  
  <?php if ( $total > $limit ) : // show this if the limit has been reached ?>
  
  <p>There are no more signups available</p>
  
  <?php else : // print the signup form ?>

  <?php // output any validation errors
  $this->print_errors(); ?>
  <?php $this->print_form_head(); // this must be included before any fields are output. hidden fields may be added here as an array argument to the function ?>

    <table class="form-table pdb-signup">

      <?php while ( $this->have_groups() ) : $this->the_group(); ?>

      <tbody class="field-group field-group-<?php echo $this->group->name ?>">
        
        <?php if ( $this->group->has_fields() && $this->group->printing_title() ) : // are we printing group titles and descriptions? ?>
        <tr class="signup-group">
          <td colspan="2">
            
            <?php $this->group->print_title() ?>
            <?php $this->group->print_description() ?>

          </td>
        </tr>
        <?php else : ?>
        <?php endif; // end group title/description row ?>

        <?php while ( $this->have_fields() ) : $this->the_field(); ?>

        <tr class="<?php $this->field->print_element_class() ?>">

          <th for="<?php $this->field->print_element_id() ?>"><?php $this->field->print_label(); // this function adds the required marker ?></th>

          <td>

            <?php $this->field->print_element_with_id(); ?>

            <?php if ( $this->field->has_help_text() ) :?>
            <span class="helptext"><?php $this->field->print_help_text() ?></span>
            <?php endif ?>
            
          </td>

        </tr>
  
        <?php endwhile; // fields ?>
        
        </tbody>

      <?php endwhile; // groups ?>
      
        <tbody class="field-group field-group-submit">
      
        <tr>
          <td class="submit-buttons">
            
           <?php $this->print_submit_button('button-primary'); // you can specify a class for the button ?>
            
          </td>
          <td class="submit-buttons">
            
           <?php $this->print_retrieve_link(); // this only prints if enabled in the settings ?>
           
          </td>
        </tr>
        
      </tbody>
      
    </table>
    
  <?php $this->print_form_close() ?>
  
  <?php endif ?>
  
</div>

If you don't want to to count all the records, you can change the query. For example, to only count the "approved" records, you can use a query like this:

'SELECT COUNT(*) FROM ' . Participants_Db::$participants_table . ' WHERE approved = "yes"'