Understanding technical terms used in SSL is essential. Here you can learn more about:
Depending on your needs or your technical pool, you may need to buy additionnal certificates or licences.
A certificate's always sold to be installed on a unique machine (physical or virtual).The additionnal licence makes then possible to duplicate an existing certificate instead of buying a new one for each different machine you may use.
Several configuration can be encountered:
A new certificate can be reissued in substitution of an existing one with the same fields but a different pair of keys*.
That can be done in those situations:
*pair of keys: Item privet key / public key elements: if a new pair of keys is needed a new CSR has to be requested.