It depends on whether you want each site to have its own domain name or not. If you dont mind, you can usually put as many sites on an account as you want using subdomains.
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 will have to pay a fixed fee for every domain you wish to add. Buying addon domain rights from your hosting provider does not actually give you a domain name, it just give you the ability to add that domain into your account. You will still have to pay for the domain name too.
If you are going to host more than one site, be careful that you dont violate your host’s Terms of Service and Acceptable Use Policy, particularly if you will be letting a third party use your account in any way. And of course, you will need to make sure you dont exceed your storage and bandwidth limits.