1. You will need to add a payment method before creating your first droplet. You will need either credit card or PayPal info.
2. You are purchasing a virtual server, so you can host as many domains as you want. You just need to either enter them in the DNS panel or use something like any DNS service provided by your domain registrar or a 3rd-party service like DNSExit.com or CloudFlare to add A, MX, etc. records for your domains.