From manual page: https://php.net/install.fpm.configuration
The documenation of "pm.max_children" should mention the case when "pm" is set to "ondemand".
I am assuming that the parameter is also valid in that case.
The way it is documented currently implies that the parameter only works when "pm" is set to "static" or "dynamic", but if I understand this correctly it also affects "ondemand".