Yes, you can register an unlimited number of domain names.
Yahoo! Domains currently registers .com, .net, .org, .biz, .info, and .us domain names. In addition to .com, many businesses register the .net and .org variations of their domain name.
Having multiple domains allows you to:
A Yahoo! Domains plan will be set up for each domain name you order through Yahoo! Each account will have a separate Domain Control Panel.
If you already own a registered domain name, you can still use it with a Yahoo! GeoCities Pro or Yahoo! Small Business plan. You will need to modify your name servers through your registrar so that Yahoo! can host your domain.
Note: This process is called redelegation — it is not a registrar transfer. You will continue to be responsible for paying registration fees for your domain name with your current domain name registrar.