If you're using Kentico CMS in a cloud, such as Windows Azure or Amazon EC2, you need to purchase a Kentico CMS license for every instance in the cloud. In the cloud environments, an instance is considered as an equivalent of server, no matter how many physical servers are involved.
Similarly to webfarms, additional instances in the cloud require only an extra Kentico CMS Base License, even if your main license is Base License with Package or Ultimate License.