1. you could host as many as you like... it just depends on how many resources the accounts will be using. once you can afford it, it is always suggested to upgrade to a dedicated server
2. staff will have to answer that one
3. private nameservers are usually set up with the account, just ns1.yourdomain and ns2.yourdomain
4. you can get a free WHMCS license when using surpass:
WHMCompleteSolution Surpass Hosting Customer Signup
5. dedicated IP addresses are an addon, they're a few bucks a month (don't remember the exact price)
Hope that clarified any questions you have