Ok first off since it is a multi-player map, you want to turn your variable into an array of 12 for all 12 players like so.
Ok, now we can make the trigger to count these kills

To get there you simply go to Actions -> Set Varriable and on the left side when you click on it, click on Arithmetic and you'll see the two values as show, put the first one your Kill_Tracker [1] and the second value + 1
Simple enough? Right, you got this.
Now your finished counter trigger should look a bit like this, except with functions for all the players.
We're not done yet, now we have to make a trigger to tell it what to make and when to stop making stuff.
Our conditions? We'll use And here, your conditions should look a bit like this: (Note: Will be using If/Then/Else, Multiple functions for each player, this is not the condition for the trigger in general.)
And bamn, we have our conditions for player one, now on to the rest of the trigger.

Turn off <Trigger> and Turn on <Trigger> tell it to stop making the level 1 spawns and start making level 2 spawns and so on.
That ends this, hope you have a better understanding of varriables and uh.. other stuff I went over.