Historical Rules · 1 June 2014
Rule 100.2b
Official source text from the 1 June 2014 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), 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.