must have physics behavior, even if its a bouncing bullet to stop.... use the "Is Sleeping" event for the objects physics
add global variables "reload"+"count" , a ball instant boolean, and a variable "stop"
Is Sleeping : set ball boolean true
while ball on layout , "stop" =0 :
-------- boolean = false :
-------- boolean = true : +1 to GV"count" +1 to "stop"
GV"count"=GV"balls" : set reload to 0
Reload = 0 : spawn ball position x,y, +1 to GV"balls", set reload to 1
this should keep a track of whats on the stage, and you can easily have the "count" shown in a text box... if your thinking of 300 balls