modify php-fpm max_children on Plesk < 17.5

cat /var/www/vhosts/system/example.com/conf/php.ini
[php-fpm-pool-settings]
pm.max_children = 100
pm.max_requests = 100

restart php-fpm version site is running
eg. for php version 5.6

systemctl restart plesk-php56-fpm.service