Constructed decks must contain a minimum of sixty cards. There is no maximum deck size. If a player chooses to use a sideboard, it may not contain more than fifteen cards.
This is one of the announcements that many Head Judges include before beginning a tournament. It holds true at any constructed tournament, and is fairly self-explanatory.
Except for cards with the basic supertype or cards with text that specifies otherwise, a player’s combined deck and sideboard may not contain more than four of any individual card, based on its English card title.
For most cards, a player may only play a maximum of four copies of a given card across their entire seventy-five. There are currently fourteen exceptions to this rule: Plains, Island, Swamp, Mountain, Forest, the five Snow-Covered variants, Relentless Rats, Shadowborn Apostle, Rat Colony and Persistent Petitioners. Note that you cannot get around this rule by using foreign or misprinted cards; the count is based on the English name of the card.