Multi-domain SSL Certificates
If you have a single app that needs to use SSL for multiple domains, you will need a multi-domain SSL certificate.
Multi-domain SSL certificates cost more than single-domain certificates. These certificates are generally priced based on the number of additional domains.
One option for a reasonably priced multi-domain SSL certificate is the PositiveSSL multi-domain certificate from Namecheap.
You do not need a multi-domain certificate if each of your domains is for a different app. In that case, you need a separate SSL certificate for each domain, not a multi-domain SSL certificate.
Example of When You Need a Multi-domain Certificate: WordPress Multisite
If you are running a WordPress multisite instance and two or more domains that are part of the multisite install need SSL, then you will need a multi-domain SSL certificate.