Thank you for the explanation Cthulu!
I already knew about for-loops in general, but extra knowledge is always good.
I used the help I got to create this script:
int Spawn_PickUnusedNumber(string &in asName, int alFrom, int alTo)
{
    int Start = RandInt(alFrom,alTo);
    int End = alTo;
    
    for(int i=Start;i<=End;i++)
    {
        if(GetLocalVarInt(asName+i) == 0)
        {
            return int(i);
        }
    }
    
    return int(-1);
} 
The idea is, I have a set of enemies. When one is spawned it's variable is set to 1. This is so that it cannot be picked while it is already active.
This Spawn_PickUnusedNumber helps me pick a number that isn't already used. If all numbers are used it'll set the number to -1, and thus...
if(iEnemy == -1)
    {
        AddDebugMessage("ENEMY NOT FOUND", false);
        return;
    } 
...ignores this spawn.
			
 
			
			
Trying is the first step to success.