[WHM or SSH] How to increase slot httpd limit?

When you login to the CPanel's WHM, goto Apache Configuration which is under Service Configuration. Next click on the Global Configuration area and towards the bottom half you can increase some of the settings there to allow more simultaneous connections to the Apache Server. Areas to consider increasing would be the Server Limit and Max Clients. I would advise you to learn exactly what it means to change any settings there, usually you can look anything up to learn exactly how it can affect your server. I typically do not put them too high unless necessary as it could eat up all of your memory and crash the server that way. So if you have alot of free memory left on the server you would probably have no problem upping some of the numbers there.