One SSL certificate per load balancer
You can add multiple domain names when creating a load balancer instance. At the same time, you will be able to add only one SSL certificate for the instance. This certificate will be used to respond to all request to that instance. Thus to protect multiple domain names you will need a wildcard certificate in case the domains are subdomains of the same 2nd level domain, or you will need a SSL certificate with SAN extension for multiple domains.