Origin: The word 'horde' stems from ordu (pronunciation:/'oɾdu/), a Turkish word meaning army. The roots of the word goes back to Genghis Kahn's highly sophisticated organization of his armies, based on the decimal system. In today's military vernacular, an ordu would be called an army corps or a field army.
The Mongol military of the 13th century was made up more or less every able-bodied male between 14 and 60, divided into the tjumen 10,000 strong, divided into the regiment, or mingghan of a 1,000 men, further divided into companies, or zuut, of 100, and finally, the groups of 10, the arav. Two to five tjumen would operate together as an ordu, or horde.
Using this system, a horde would count between 20,000 and 50,000.
Now, if only there was a way to answer the age old question: how many boards could the Mongol hordes hoard if the Mongol hordes got bored...
Source: Wikipedia, The Realm of the Mongols, and Elite Series #30, Atilla and the Nomad Hordes, Osprey Publishing.