A "node" is a single installation of an operating system running on a machine (virtual or physical) that will utilize the Software. The "node" is counted when used by Developers, Testers and in Production.
If there are multiple virtual machines running on a single physical machine, each virtual machine will be counted as a Node while the physical machine will not be counted
Inactive Nodes like those used for disaster recovery or the passive Nodes in an active/passive cluster will not be counted as an additional license per Node
If a Node is used for the purpose of Automated Builds or Automated Tests, and is not otherwise used by Developers, Testers, or for Production Use, you will not be required to purchase a separate license for it.
Yes, you can buy licenses for any number of months up front.