If I play Counter Strike in a combat versus multiple units, does it block the damage from all units or just one, does the opponent choose what unit does the damage that gets blocked?

When you play Counter Strike, it prevents all damage that would be dealt to the chosen unit in that specific instance.

Because combat damage is dealt simultaneously (Rule 443.1.d.1.a), all damage being dealt to your unit at that moment is considered a single event. Therefore, Counter Strike will prevent all of that damage, regardless of how many units are attacking it.

To answer your specific questions:

• Does it block damage from all units or just one? It blocks all damage dealt to the chosen unit during that combat step, as all combat damage is dealt simultaneously.
• Does the opponent choose what unit does the damage that gets blocked? No. The effect is not tied to a specific attacking unit; it is tied to the unit you chose to protect. It prevents the damage that unit would take from any source during that next damage event.

Summary: When you play Counter Strike, you choose one of your units. The next time that unit would take damage (which, in combat, is the simultaneous damage step), all of that damage is prevented.