Ok, assuming that my webhosting business grow from 1 server to 2-3 servers. Do I need to get additional license for CPanel and billing system (e.g. PerlBill, PHPManager or ModernBill) for my other 2 new servers?

Cpanel yes, the other two, most likely no. You shouldn't need more than one license unless you plan to use a separate instance of your billing system for each server, which would be kind of silly. Both phpmanager and modernbill support multiple servers. :)