I thinks its its gotta be the no. of factors 120 has,including the 1 point at the beggining...
Since 120 has 15 factors..(1,2,3,4,5,6,8,10,12,15,24,30,40,60,120)..+ 1 point in the start,so that makes it 16 points..
in every 'n'th round 1 will get added to box no. 120 where n=factor of 120 :thumbsup: