If you are using cPanel with Apache 2.0, you will find out your Apache max clients limit is set to 256 only and this is a little harder for a normal server to serve their user.
To make this setting done, you may refer to the step as below.
1. Log into WHM.
2. Select Service Configuration and click on the Apache Configuration.
3. Select Global Configuration.
4. Change the ServerLimit and MaxClients limit to 512, save it.
5. Rebuilt the Apache.
With the above term, make sure that you have enough CPU and Memory resource or your server will down because of limited resource.