box collider trigger question

ok so i kinda have 2 questions. ive been able to figure out how triggers work with box colliders and send messages and look at all that fun stuff. but now the hard parts. 1st question im wondering if its possible to have a collider trigger after walking across it like 2 times? for example i want to be able to walk across it once with nothing happening, then when u walk over it again thats when it triggers whatever.
next question is is it possible to make them appear and dissapear at certain times. another example being like i have a level that requires you to collect keys. when i pick up the first key i want to be able to delete all those box colliders i had before and then make a whole new set of them. then again for the second key and so on.
i hope i made sense in all that, if not ill try to explain it more.

sorry if its kinda long :frowning:

Question 1: Create an int count so it will up the count each time you walk over the collider. If the desired count is met, trigger the effect.

Question 2: Im not sure what you mean by deleting all the box colliders from before. But, instead of deleting the colliders, just turn the activate/deactivate through object.renderer.enabled = true/false.

void OnTriggerEnter(Collider other)
     if(other.gameObject.tag == "Tag Name")
          other.renderer.enabled = true;