Below is an extract of an article written by Karl Rixon :
http://www.karlrixon.co.uk/articles/apache/configure-apache-maxclients-via-whm/
1. Log into WHM
2. Under Service Configuration in the left menu, select Apache Configuration.
3. Click Include Editor
4. In the Pre-Virtualhost Include section, choose All Versions from the drop down.
5. Enter your config settings.
[…]
ServerLimit must come before MaxClients !