No, one SSL certification will not cover all the domains on a server.  We do offer a SSL certification that will cover all sub-domains of a domain.
To answer this question it is important to understand how your websites are set up and how SSL Certifications work.
If each site is set up as their own domain (Examples: www.domainname.com   www.mydomain2.com) each site would need their own SSL Cert.
If each site is set up as a sub-domain (example: billing.domainname.com, support.domainname.com, www.domainname.com) the site and all sub-domains could be covered by one SSL Cert.