What I would do is create a variable to track the number of objects with that tag and when =0 send event to move on. You could do this by anytime one of the objects is spawned, just add 1 to the variable (probably use an Int). If the game starts with the pbjects already spawned, go into the variables menu and set the variable to say 50 if that is how many. To remove, on the same state that you destroy the object (I'm assuming you destroy it) just subtract one. And on another object, not the tagged objects set a constant Int compare where you test if the variable is equal to 0, then send an event.
Hoped this helped! Let me know if you have questions!
Thanks!