Well, the obvious answer is to start smaller. Nevertheless, for the purpose of your question, let’s just look at the current size of our genome. The average number of new mutations in the genome of each human is 60. The overall genetic variation between humans is several million differences. A fairly small number of these differences are being selected upon strongly, a larger proportion are subject to weak selection, and the rest (neutral variations) are just ‘drifting’ with the fortunes of their carriers.
Obviously, the number of possible “unique animals” that can be formed using our genetic code is enormous! How do you think we should mathematically quantify this proportion? And how can we track through time the changing probabilities of viability as the complexity of our organism population grows?