Keritot 2 - Why Count Cases?
There are thirty-six actions, for which one becomes liable to be cut off from his spiritual source. These include relations with one's mother, one's father's wife even if she is not his mother, one's sister, and one's daughter-in-law. They also include cohabiting with animals, with a woman who did not go to a mikva, eating bread on Passover, and violating Shabbat. As a rule, if one does it by mistake, he needs to bring a sin-offering, but if he does them on purpose, then he may be cut off.
Exactly what being cut off means depends on the person, on his action, and on his other deeds. He may die early or see his children die, or he may live to an old age, but loose connection with the Creator, in this world and perhaps even in the next.
Any time that a rule mentions the number of cases, it means to teach us something extra, since we could count ourselves. In the case of thirty-six acts above, it wants to tell us that if he did many different acts while forgetting that they are forbidden, he needs to bring a sacrifice for each one.
Art: Pieter de Hooch Interior with a Mother delousing her child's hair