first make a global variable called sprite count then go to the action "for each" under the system object which allows you to say "for each "sprite" add 1 to sprite count.
This will add 1 to the variable for each enemy every tick..
After 1 second with 10 enemies at 60 fps the variable would be 600
resetting it to 0 every tick before repeting the for each event would solve that ofcourse,
but enemy.count will allways stay at the amount of instances of the enemy object present, so using that would be much easier..