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The Thirteen are a prominent group of traders within Qarth. They fight for dominion over Qarth with the Tourmaline Brotherhood, the Ancient Guild of Spicers and the Pureborn. There are thirteen members. Xaro Xhoan Daxos is a member of the Thirteen. The Thirteen own roughly a thousand ships.
A Clash of Kings
After Daenerys enters Qarth Xaro tells Pyat Pree that the Thirteen shall set a crown of black jade and fire opals upon her head. When she settles into Xaro's home he tells her that Thirteen will come and do her homage, and all the great of Qarth. The Thirteen's crown of black jade and fire opals is not mentioned again and it is the Tourmaline Brotherhood that give Daenerys Targaryen her crown.
Daenerys Targaryen asks for ten ships from each of the Thirteen which would total one hundred and thirty ships. She is refused. Only Xaro's influence prevents the Thirteen from adding their voices to that of the other trade guilds demanding Daenerys's expulsion and possible assassination after the destruction of the House of the Undying.
A Dance with Dragons
Xaro Xhoan Daxos sails to Meereen with a fleet of thirteen galleys, and men to pull the oars, to gift to Daenerys so that she can sail home to Westeros. Dany thinks to herself that Xaro is one of the Thirteen and no doubt had convinced each of his fellow members to give up one ship. She knows the merchant prince to well to think that he would sacrifice thirteen of his own ships.