How many hearts do a whale have?

You may be confusing "heart" with "stomach." No mammal has more than one heart. Several animals have more than one stomach, or multi-chambered stomachs. These are called ruminants. a Ruminants are mammals in the order Artiodactyla. They predigest their food in the first stomach, regurgitate it, re-chew it (it is called cud now) and re-swallow it.