This comes up a lot. You get as many as it takes to finally miss the target box line.
The rule is there because it simplifies the rules dramatically and allows the shooter to make a good decision about what to do through the scope. Without the rule…if you land in between the boxes you have to be able to assign the shot to a specific box. It becomes complicated to come up with a rule to do that fairly. This is simple.
Lots of guys seem to worry that someone could hit the line 5 times, then finally get an x. That could happen, but it’s highly unlikely that someone who is hitting the box so often will be in the running for anything.
Hope that makes sense. N50 was designed to be easier on newcomers but still allow the experienced shooters to be very competitive.
Mike