You may only have one x10 hosting account, period.
You may add up to three domains to your account as 'add-on domains' . The content for each is kept in a separate folder of public_html. Along with your primary domain, that would make 4 separate sites you can host from one account.
Disk space and bandwidth will be unmetered starting June 1, so that should not be a problem unless you abuse it.
For more email accounts/databases/etc, you can purchase the double-all package for $5 for the lifetime of the account. You can do it twice for 4X ... 12 databases, 12 FTP accounts, 20 emails (or go thru Google if you need more), 12 add-on domains.