In Apache 2.4 (which is in Centos 7 and Ubuntu 14.04 default) the mpm_worker MaxClients has been replaced with MaxRequestWorkers.

In Ubuntu 14.04 you can see the below in /etc/apache2/mods-enabled/mpm_prefork.conf

<IfModule mpm_prefork_module>
StartServers 5
MinSpareServers 5
MaxSpareServers 10
MaxRequestWorkers 150
MaxConnectionsPerChild 500
ServerLimit 300
</IfModule>

In Centos 7 however there is NO configuration for this, which means it’s at the default value of 256 unless set by the user. This also means that StartServers is set by default to 3, MinSpareServers is set to 5, and MaxSpareServers is 10.
To configure Centos and set some limits, you need to edit this file /etc/httpd/conf.modules.d/00-mpm.conf appending the above content.

Always verify with apachectl -t or  httpd -t if all is ok before reloading/restarting Apache.

NOTE1: ServerLimit is not in the default configurations for either Centos 7 or Ubuntu 14.04 which means that if you set MaxRequestWorkers above 256, you must remember to add ServerLimit!

NOTE2: MaxConnectionsPerChild set to 0 on Ubuntu 14.04, and 0 is also the default in Centos 7. This means that on both the Apache processes will not expire. This is going to be bad for users who like to set their php memory_limit to 1G!