By design, there impossible to 'limit' (including 'fastcgi children') for admin.
Need using custom.php53-fpm-pool.conf.tpl with modified:
if ($maxchildren) {
to:
if ($user === 'admin') {
$maxchildren = '12';
} elseif ($user === 'apache') {
// using by cp and webmail
$maxchildren = '6';
}
if ($maxchildren) {