during a combat showdown i use Counter Strike in my unit agains 2 units, How it works?

Yes — Counter Strike prevents all combat damage to your chosen unit, from both attackers.

All combat damage is dealt simultaneously as a single damage event (Rule 465.2.c.1.a), so the "next time that unit would be dealt damage" prevention from Counter Strike applies to the entire event — damage from both units is prevented at once (FAQ #8527, #9605).

Your unit survives the damage step. If the attackers also survive (no damage assigned to them, or their damage was prevented too), the Combat Cleanup will recall the attackers to their base since defenders are still present, resulting in "No Result" (Rule 466.1.a.2, 466.3.d). Counter Strike is a [Reaction], so you can play it during the combat showdown even while the Chain is resolving.