In Risk, what is the minimum army units you must leave behind in a territory after attacking?

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Which twins, children of Zeus and Leda, are known together as the Dioscuri? Who created the popular Broadway musical "Hamilton"? What European city hosts a famous annual Christmas market known as Christkindlesmarkt? Who was known as the Liberator or El Libertador? Who was the Scandinavian warrior who conquered Normandy around 911 CE?