One of the main User interface and gameplay variables is the amount of units that a player can control at a time.
It limits the amount of targetted damage output, the amount of workers you run away from the Psi storm, the amount of Hydra's you patrol in at a time.

Karune stated that at this point, the amount of units selected is unlimited, I do not believe that this will be the case, and we will see 36 units per selection.

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Admiral Anderson Mccutcheon
Inspector General
TerranoZergus Project