NGINX products are licensed “per instance”. Each subscription allows you to run one single instance of the software for the subscription period.
An instance is a single copy of the software running on a single physical or virtual server. If you run two copies of the software on the same physical or virtual server, that counts as two instances. When you run two copies of the software on two different physical or virtual servers, that also counts as two instances. Most disaster‑recovery (DR) deployments require two instances per location, even if one instance is “passive” (running but not receiving traffic).
If you run a “hot/cold” DR deployment where one server is powered down (that is, the NGINX software is not running), and powers up only when the primary server fails, then you only need one subscription because only one copy of the software is running at any given time.