Depending on the type of SSL Certificate, you can secure from one to 100 domains.
Comodo Multi-Domain SSL Certificates secure from three to 100 domain names.
Geotrust True BusinessID cover five to 25 domains.
Verisign and Thawte Certificates secure from one to 25 domains.
QuickSSL Premium is the only SSL Certificate that secures subdomain names only. One QuickSSL Premium can secure five host names at once within the same domain name.