Each license regardless of level will come in a 4 core package. (Enterprise 4 core) or (workgroup 4 core).
To make it easy, each ArcGIS Server will come in 4 core blocks, and this also goes for any extension. (e.g. GeoEvents) would also require 4 core.
If you are in an Enterprise environment, the cost will be half of the cost you spend in production in licenses for standing up a Staging Cluster.
If you are using a Dev environment, use EDN.
If you are having an Active DR, then the cost will essentially be (double) as your live production web stack.
If your DR is passive and NOT in the same datacenter, the licences are still double. There are no license additions if DR is passive and within the same datacenter.