The enterprise plan is per jsreport instance. If you have jsreport in 3 containers in 3 zones and it boots up in all of them, you need 3 licenses.
The scale license is here to simplify such scenarios. You get one license key and you don't have to worry how many instances you run.
We may consider in the future an extra license plan for the (hot) servers that aren't actually used and just ready to jump in.
However, for now, we have it in the simple two options form
enterprise license - per instance
enterprise scale license - infinite