Not familiar with a dual n-back game.
But it sounds like you should use an incrementing counter variable.
| Global number NthTime = 5
+ CertainObject: On collision with Square
-> Square: Add 1 to collisionCount
+ Button: On clicked
+ System: Square.collisionCount%NthTime = 0
+ System: Square.collisionCount ? 0
-> Function: Call "getPoints" ()
For reference, % gives you the remainder after division, which is what you need to use for every x overlaps. So if it is set to 5, that condition will be true when collisionCount = 0,5,10,15 ect. (then add an additional condition to make it not true when 0)
You might need to add a system to disable the event after points have been awarded for each tier.