By default, all shared webhosting and reseller accounts have only one shared IP address and no dedicated IP address. A dedicated IP change requires 4 hours to resolve. Your site may appear to be down during that time. You will NOT need to change your name servers.
Shared Web Hosting has only one cPanel. So you can have only one dedicated IP. If you choose the dedicated IP, all of your addon and subdomains will use the dedicated IP. Emails will still use the shared IP. As a Resellers, you can buy a dedicated IP and make it shared IP for all cpanels you host under your reseller account.
Dedicated IP address is needed to install SSL certificate. If you don't have dedicated IP address for your account then you have to purchase it or else the SSL certificate will not work.