If you are going to purchase an SSL Certificate that should be installed in the future on several servers, you need to pay attention to the certificate’s features – there should be mentioned Unlimited Server Licences.
Taking this into account, we recommend to purchase either Comodo or GeoTrust Certificates, in case you need to have them installed on several servers. SSL Certificates of any validation level issued by these Certificate Authorities can be installed on multiple servers.
At the moment this feature is not available for Thawte and Symantec Certificates for free. According to their Policies an additional licence should be purchased at additional cost.