Its one of those cards, that you can only have one per deck.
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Typically you put those "just-in-case" cards into your side deck; cards to respond to a variety of deck types. For example, you might include Shadow-Imprisoning Mirror in your side deck for when you go up against a DARK-attribute deck. Including that card in your main deck could prove to be less than optimal if your opponent is not using many DARK monsters.Another use of the side deck (albeit a rare one) is to radically change your deck. The trick here is that your opponent has already seen your deck once and is using their side deck to counter yours. You can throw him/her off by constructing your deck and side deck in such a way that you know which cards to swap out to make your deck run differently (i.e. taking out your effect damage cards to include out-of-play cards).
You may have up to three in your main deck and side deck combined.
The maximum is 60 in the main deck.
You can have three Dian Keto in your main deck and side deck combined.
Well, the minimum in a yugioh deck is 40 cards maximum 80cards. Most people try to have a smaller deck over a larger deck, to get cards they need. About a 40-50 card deck is good.