It would be somewhere under /etc/apache2/ (in mods-available maybe?) in debian, and I think it's /etc/httpd/ on centos. Or simply `find /etc/ -name mpm_prefork.conf`.
Thanks, I found it now. I must have overlooked this folder "/etc/apache2/mods-available". One little question, is the ISPConfig setting "PHP-FPM pm.max_children" the same setting as the "MaxRequestWorkers"? If not can we set the "MaxRequestWorkers" directly in ISPConfig?
No, the former is a setting of a php-fpm daemon, the latter of the mpm_prefork apache module. I doubt it, but just edit the .conf file directly. (Side note: you probably don't even want to run mpm_prefork nowadays, as it precludes http/2.)
What would be the best way to fix this "MaxRequestWorkers" error. I have installed ISPConfig 2 years ago with this guide "The Perfect Server - Debian 9 (Stretch) with Apache, BIND, Dovecot, PureFTPD and ISPConfig 3.1".
Find that setting in the .conf file, or add it if needed, increase it to what you need, and restart apache.
