代码：

function limit_user_registrations_per_ip($errors, $sanitized_user_login, $user_email) {
    $ip = $_SERVER['REMOTE_ADDR'];
    $registered_users = get_users(array('meta_key' => 'ip_address', 'meta_value' => $ip));
    $allowed_users = 5; //设置每个IP地址允许注册的最大用户数量
    if(count($registered_users) >= $allowed_users) {
        $errors->add('ip_address_limit', __('Error: You have reached the maximum number of registrations from your IP address.'));
    }
    return $errors;
}
add_filter('registration_errors', 'limit_user_registrations_per_ip', 10, 3);

修改代码中的$allowed_users变量，设置每个IP地址允许注册的最大用户数量。