You set your Quotas in you webhosting cpanel then link it to your proudcts in whmcs
Adding a cPanel Server
The following steps guide you through the process of setting up a server in WHMCS for cPanel Hosting Accounts.
1. Begin by going to Configuration > Manage Servers
2. Click Add New Server
3. Enter the server name, IP address, select the server type "cpanel" from the dropdown menu and enter the username and password for cPanel/WHM
4. Click Create Server to complete the process and add your new cPanel Serve