as of 2011, mystical space typhoon has been switched to semi-limited.
(Link to Limited and Forbidden Lists: http:/www.yugioh-card.com/en/limited/#limited_trd)
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You may have up to three in your main deck and side deck combined.
The maximum is 60 in the main deck.
Its one of those cards, that you can only have one per 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.