Limit the number of total sign ups for a given membership level

pmpro-limit-members-per-level.php

<?php
/*
	Set a maximum number of members allowed to register for a membership level.
	
	Add this code to a plugin for PMPro Customizations.

	Set the "Maximum" for a level on the Memberships > Membership Levels > Edit Level admin page.
*/
function pmproml_pmpro_save_membership_level( $level_id) {
	if( $level_id <= 0 ) {
		return;
	}
	$limit = $_REQUEST['pmpro_member_limit'];
	update_option('pmpro_limit_'.$level_id, $limit);
}
add_action( 'pmpro_save_membership_level', 'pmproml_pmpro_save_membership_level' );

function pmproml_pmpro_membership_level_after_other_settings ( ) {
	?>
	<h3 class="topborder"><?php _e('Membership Limits', 'paid-memberships-pro');?></h3>
	<table class="form-table">
		<tbody>
			<tr>
				<th scope="row" valign="top"><label for="pmpro_member_limit"><?php _e('Maximum', 'paid-memberships-pro'); ?></label></th>
				<td>
					<?php
						if( isset( $_REQUEST['edit'] ) ) {
							$edit = intval( $_REQUEST['edit'] );
							$limit = get_option( 'pmpro_limit_' . $edit );
						} else {
							$limit = "";
						}
					?>
					<input type="text" name="pmpro_member_limit" id="pmpro_member_limit" size="6" value="<?php echo $limit; ?>" />
					<p class="description"><?php _e('Set the maximum number of members for this level.', 'paid-memberships-pro'); ?></p>
				</td>
			</tr>
		</tbody>
	</table>
	<?php 
}
add_action( 'pmpro_membership_level_after_other_settings', 'pmproml_pmpro_membership_level_after_other_settings' );

function pmproml_pmpro_registration_checks( $value ) {
	global $wpdb;
	
	$pmpro_level = pmpro_getLevelAtCheckout();
	$level_id = $pmpro_level->id;
	
	//get the maximum number of members allowed in this level
	$limit = get_option( 'pmpro_limit_' . $level_id );
	
	//get the count of members in this level
	$sql = "SELECT COUNT(*)
            FROM {$wpdb->pmpro_memberships_users}
            WHERE `status` LIKE 'active' AND `membership_id` = ". esc_sql($level_id);
	$member_count = $wpdb->get_var($sql);
	
	//compare the count of members to the maximum number of members allowed in this level
	if($member_count >= $limit) {
		global $pmpro_msg, $pmpro_msgt;
		$pmpro_msg = __('Membership limit has been reached for this level', 'paid-memberships-pro');
		$pmpro_msgt = "pmpro_error";
		$value = false;
	}
	
	return $value;
}
add_filter( 'pmpro_registration_checks', 'pmproml_pmpro_registration_checks' );