I believe the title speaks for itself.
At some point in the past year I played some of the remade Brood War maps on Battle.net, which featured limited control groups to simulate the old StarCraft gameplay.
I can't really wrap my head 'round whether the developer how made the system, whether it'd be through the use of triggers or the Data editor.
It can be done in triggers. Create a trigger with Event Unit is Selected. Add a condition for number of units selected for triggering player <=12. Then pick each unit in selected units for triggering player and deselect them until number of selected units is under 12. This is assuming if I select 12 units at once, the trigger fires once. If it triggers once for ever unit I select, then it would be a simpler matter of checking the condition, if it passes, deselect the unit.