I found it in my notes. It is the combination rule in the rules of counting
C=n!/[(n-k)!k!]
C is the number of combinations
n is the number of morphs you have
k is the selection set you are using
For example, if you have 15 genes in your collection and you assume you can only get 5 into a snake, that means you have 3003 possible combinations. If you assume 6 you have 5005 possible combinations.