they're counting the 'animal' mutations, as opposed to the 'ĝheist' ones which are yellow. the vast majority of mutations are ĝheist, though, so it's entirely possible to play through the entire game and not get a single new 'animal' mutation.
there's brief explanations for how to get various tfs in the 'walkthrough' section on the title page; the ones marked as 'animal' in the code are 'knotted dog dick', 'six nipples', and 'coarse fur across your forearms and calves'.
