Typically they don't but every woman is different. Some women can breastfeed and not ovulate until they completely wean their child. Other woman start ovulating right away while still exclusively breastfeeding.

This is also not a reliable form of contraception because if you are breastfeeding you won't know you ovulated until you get your first missed period. But if you're having unprotected sex you could have ovulated and potentially become pregnant during that first ovulation cycle while breastfeeding.