Combinatorial can be quickly calculated primarily using factorials. 4 possible options, each where you're picking all 4 exactly once is 4!. The reasoning is pretty intuitive, when you start selecting there are four options, when you go to pick the next one there are 3 left in the pool, then 2, and finally 1. This turns into 4 * 3 * 2 * 1 = 24.
What is the generalised formula for calculating this, given the number of nodes but also edges need to be considered.
It doesn't appear to be explained in the article. I think it may be a factorial?