have you tried making a sprite make it 10px width and height. Input a variable 'Enemies_count'. and put the number of enemies in it.
when you destroy enemies -1 from variable 'Enemies_count'. And then run an event like this 'Enemies_count=0' and do what ever action you want to do.
And if you want something like a specific item to spawn after two and not that third one are killed then just make another variable Item_Timer
and run the same action of -1 again for on destroyed.