A single IP# address can be dedicated to a user account, and then the user account can use it for multiple sites.
However, as Jon points out, a standard cert can only be issued in one domain, and any other domain attempting to use that cert will come up with an error in every user's browser each time s/he attempts to use the cert.
You can buy a wildcard cert for, for example, *.example.com, and then you can have secure access to sub1.example.com, and sub2.example.com, but those certs are quite expensive.
Jeff