Based on the answer from simc: simulationcraft/simc#3423
Right now desired_targets is translated into Enemies(tagged=1). From what I understand it is the amount of enemies we have damaged already.
However, this does not seem to correspond to the actual meaning of desired_targets.

2 options:

• count the amount "boss" units (which are hostile); but what if they are spread out?
• hardcode it to 1

Any other options?

• an option in the GUI.

I actually just made a drop down box manually in the feral script. There's not really a good way to do it. Which scripts actually use it?

a quick search in all the simulationcraft scripts:

• havoc demon hunter
• feral druid
• arms warrior
• fury warrior

This should just be hardcoded to 1 in the default scripts.

Another possibility is to translate it to a custom function DesiredTargets() whose value is taken from a drop-down list created via AddListItem() and which defaults to 1. This would allow for some in-game customization of the value.

I would suggest the drop-down choices be 1, 2, or 3.