Wildcard SSL certificates can cover one main domain (www.domain.com) and an unlimited amount of subdomains (mail.domain.com, login.domain.com, test.domain.com, etc.). Multi-domain (SAN) SSL certificates can cover multiple domains on just one certificate. For example, Symantec and Thawte multi-domain certificates can cover up to 25 domains, whereas Comodo certificates can cover up to 250 domains with just a single SSL certificate. GeoTrust multi-domain certificates can cover anywhere from 25 to 250 domains, depending on the type of certificate you order.