An Idiot's Array was a rare hand that can be achieved in some versions of sabacc. It consists of The Idiot, a two of any suit, and a three of the same suit (though in some versions, the two and three were not required to be suited). Rather than being added together, which under normal sabacc rules would give a total of five (2 + 3 plus the Idiot's value of zero), it was read as 23. An Idiot's Array trumps even Pure Sabacc (a score of positive or negative 23).
Vuffi Raa explained the Idiot's Array to Waywa Fybot during a journey through the Oseon system, while attempting to teach the avian Imperial officer how to play sabacc. Fybot decried the Idiot's Array as "ridiculous." 
Two Idiot's Arrays were seen in the game that Darth Bane, known as Dessel at the time, played against a Republic soldier, an Ensign, while he was still on Apatros some time before he joined the Sith Army. Darth Bane won this game of sabacc in sudden demise with an Idiot's array hand of 2, 3, the Idiot, nine, and nine.
There can be only two Idiot's arrays because there are only two idiots in the deck.