paragraph: Alexander was born on the sixth day of the ancient Greek month of Hekatombaion, which probably corresponds to 20 July 356 BC, although the exact date is not known, in Pella, the capital of the Kingdom of Macedon. He was the son of the king of Macedon, Philip II, and his fourth wife, Olympias, the daughter of Neoptolemus I, king of Epirus. Although Philip had seven or eight wives, Olympias was his principal wife for some time, likely a result of giving birth to Alexander. Several legends surround Alexander's birth and childhood. According to the ancient Greek biographer Plutarch, Olympias, on the eve of the consummation of her marriage to Philip, dreamed that her womb was struck by a thunder bolt, causing a flame that spread "far and wide" before dying away. Some time after the wedding, Philip is said to have seen himself, in a dream, securing his wife's womb with a seal engraved with a lion's image. Plutarch offered a variety of interpretations of these dreams: that Olympias was pregnant before her marriage, indicated by the sealing of her womb; or that Alexander's father was Zeus. Ancient commentators were divided about whether the ambitious Olympias promulgated the story of Alexander's divine parentage, variously claiming that she had told Alexander, or that she dismissed the suggestion as impious. On the day that Alexander was born, Philip was preparing a siege on the city of Potidea on the peninsula of Chalcidice. That same day, Philip received news that his general Parmenion had defeated the combined Illyrian and Paeonian armies, and that his horses had won at the Olympic Games. It was also said that on this day, the Temple of Artemis in Ephesus, one of the Seven Wonders of the World, burnt down. This led Hegesias of Magnesia to say that it had burnt down because Artemis was away, attending the birth of Alexander. Such legends may have emerged when Alexander was king, and possibly at his own instigation, to show that he was superhuman and destined for greatness from conception. In his early years, Alexander was raised by a nurse, Lanike, sister of Alexander's future general Cleitus the Black. Later in his childhood, Alexander was tutored by the strict Leonidas, a relative of his mother, and by Philip's general Lysimachus. Alexander was raised in the manner of noble Macedonian youths, learning to read, play the lyre, ride, fight, and hunt. When Alexander was ten years old, a trader from Thessaly brought Philip a horse, which he offered to sell for thirteen talents. The horse refused to be mounted and Philip ordered it away. 
question: Olympias was the fourth of how many wives?
answer: Nine