WHMCS = Billing system
cPanel/WHM = Server Control Panel
WHMCS can provision as many shared accounts across as many cPanel/WHM servers you need. In the latest versions you have server groups where it will evenly distribute account provisioning when a new client signs up.
So in short, you can have one system/account/server/vps with WHMCS. Which can technically provision on 1, 2, 3, 500, 5000+ cPanel/WHM servers, etc.
use hsphere control panel, it will all solve the problems
The main advantage of HSphere is that its multiplatform and works on the cluster concept where you have different name servers, web servers, database servers, etc. Very good if you have a large number of servers and would like to split tasks. You have one central CP server which communicates with the other servers
The biggest advantage of H-sphere is its scalability and the total cost of ownership. System Administration, Billing, Support, Marketing ...nothing left for a hosting company
If you're using WHM/cPanel for your server, a lot of software will automate signup, not only WHMCS (although its probably one of the best), stuff like the latest versions of CE, Modernbill and (I believe) ubersmith will do it to, also handling auto suspension etc which saves a lot of time