Number of paths for different points

Question

I have a data where there are possible paths for each point like,

1 = [7, 8]
2 = [8]
3 = [5]
4 = [5, 6]
5 = [11, 12]
6 = [12]
7 = [9]
8 = [9, 10]
9 = [15, 16]