Historical Rules · 13 November 2015
Rule 100.2b
Official source text from the 13 November 2015 Rules release
In limited play (a way of playing in which each player gets the same quantity of unopened Magic product such as booster packs and creates his or her own deck using only this product and basic land cards), each deck must contain at least forty cards. A limited deck may contain as many duplicates of a card as are included with the product.