It depends on whether you want each site to have its own domain name or not. If you don't mind, you can usually put as many sites on an account as you want using subdomains, i.e. site2.mydomain.com.
Depending on your hosting plan's features, you may be able to host multiple websites on your account by default. Otherwise, you can purchase the ability to use addon domains. You'll have to pay a fixed fee for every domain you wish to add.
Buying addon domain rights from your hosting provider doesn't actually give you a domain name; it just gives you the ability to add that domain to your account. You'll still have to pay for the domain name, too.
If you're going to host more than one site, be careful that you don't violate your host's Terms of Service and Acceptable Use Policy, particularly if you'll be letting a third party use your account in any way. And of course, you'll need to make sure you don't exceed your storage and bandwidth limits.