Per the Wiki page for AQHA, the American Quarter Horse Association is the largest breed registry in the world, with more than 5 million American Quarter Horses registered.

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At the 2011 Jockey Club Round Table in Saratoga Springs, New York, Michael Lamb and Dan Singer of McKinsey and Company presented the results of a study commissioned by the Jockey Club, which is the breed registry for Thoroughbred horses in this country, Canada, and Puerto Rico.

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