Depending on the type of SSL Certificate, you can secure from one to 100 domains.
The default price includes the limited amount of domains. The additional domains require additional payment. The price for additional slots can be checked on the page with the details of the particular certificate.

Comodo Multi-Domain SSL Certificates secure from three to 100 domain names.

Geotrust True BusinessID cover five to 25 domains.

Symantec and Thawte Certificates secure from one to 25 domains.

QuickSSL Premium is the only multi-domain SSL Certificate that secures subdomain names only. One QuickSSL Premium can secure five host names at once within the same domain name.