I would place a massive trigger, which covered all OPFOR:
Activation: OPFOR PRESENT ONCE
{ [_x] execVM "addDeathCounter.sqf" } forEach thisList; totalEnemies = count thisList
(if you add more OPFOR during the mission, then you'd have to run addDeathCounter.sqf for each of them too).
_enemy = _this select 0;
if (isNil "numKilled") then { numKilled = 0 };
_enemy addEventHandler ["KILLED", {
numKilled = numKilled + 1;
}];
I don't know how you end your mission, so I can't help you with knowing when to use this (sadly, you can't put dynamic text into the debriefing):
[playerSide, "HQ"] sideChat format ["Well done, %1! You neutralised %2 OPFOR out of %3.", name player, numKilled, totalEnemies];