You don't need an apostrophe for marking plural. "The king wants two pizzas". You use it to mark property: "The pizza's taste was to the king's liking." Or you use it to mark where something is cut when pronounced: "I'm fond of pizza." You also use it when talking about someone's house or shop: "Another king cameover to the king's". And you use it to mark property when the nomen ends with an s (or a similar sound): "Both kings' pizza were delicious."