You are using the unit version of script control. You need to use the unit group version. That way it'll be for all the units and not just the very last one.
If you're using a create unit action, to reference those units as a unit group, use the function "Last created units".
It would look something like this:
Test spawn units
Events
Game -Map initialization
Local Variables
Conditions
Actions
Unit -Create 10 Marine for player 1 at Start Location 001 using default facing (No Options)
AI -Enable script control for (Last created units)
You are using the unit version of script control. You need to use the unit group version. That way it'll be for all the units and not just the very last one.
If you're using a create unit action, to reference those units as a unit group, use the function "Last created units".
It would look something like this:
In triggers you could use the action "enable script control"