It depends on which provider you are buying from. Many providers such as ssl.com allow you to install the same ssl certificate onto as many servers as you like so the number of server licenses will not even be asked. In fact, at ssl.com, we allow you to have multiple concurrent private keys so that your security is increased even more.
On the other hand if you are asked about the number of servers, then you need to put down how many servers you intend to install the ssl certificate on. But as mentioned earlier, at ssl.com, we allow unlimited servers licenses so we do not ask you how many servers you intend to install on.