How do I change the php-fpm settings for my domain?

This only applies to VMs running Plesk 12 or newer, and have PHP set to run as an FPM application.  This is specified in Plesk under:

Domains > domainname > PHP Settings

PHP-FPM is generally quicker and less resource-intensive as running in fast\fcgi mode.  However if you wish to change the default limits Plesk sets on PHP-FPM sites then you need to create a custom php.ini file for any domains you want to change the limits to.

This is done by creating a blank file named php.ini in the following location:

/var/www/vhosts/system/mydomain.tld/conf/php.ini 

where mydomain.tld is the name of the domain whose settings you wish to change.

Then to have plesk read in your changes run the following command via SSH:

/usr/local/psa/admin/sbin/httpdmng --reconfigure-all

For example if you wanted to change the max_children limit you would put the following in to this file:

[php-fpm-pool-settings]
pm.max_children = 20

A limit of 20 should be safe enough for our Business VMs or higher with a handful of busy sites, or a larger number of smaller websites.