STARTUP THE GAME

Deck construction

Players choose a legendary creature/planeswalker as the commander for their deck. Some cards may have rules text that overrides this restriction. (e.g. legandary creatures with ability "choose a background")
A card’s color identity is its color plus the color of any mana symbols in the card’s rules text. A card’s color identity is established before the game begins, and cannot be changed by game effects. The cards in a deck may not have any colors in their color identity which are not in the color identity of the deck’s commander.
A Commander deck must contain exactly 100 cards, including the commander plus up to 10 sideboard cards and it must adhere to color identity and singleton rules. So the maximum size is 110 cards.
With the exception of basic lands, no two cards in the deck may have the same English name. Some cards (e.g. Relentless Rats) may have rules text that overrides this restriction.

Play Rules

Players begin the game with 25 life.
Commanders begin the game in the Command Zone. While a commander is in the command zone, it may be cast, subject to the normal timing restrictions for casting. When you cast it or activate its specified ability from the command zone, put a tax counter in the command zone. Its owner must pay (2) for each tax counter in the command zone; this is an additional cost.
If a commander is in a graveyard or in exile and that card was put into that zone since the last time state-based actions were checked, its owner may put it into the command zone. If a commander would be put into its owner’s hand or library from anywhere, its owner may put it into the command zone instead. This replacement effect may apply more than once to the same event.
Being a commander is not a characteristic [MTG CR109.3], it is a property of the card and tied directly to the physical card. As such, “commander-ness” cannot be copied or overwritten by continuous effects. The card retains it’s commander-ness through any status changes, and is still a commander even when controlled by another player.
If a player has been dealt 21 points of combat damage by a particular Commander during the game, that player loses a game.
Commanders are subject to the Legend rule; a player cannot control more than one legend with the same name.

Players start each with seven (7) cards in hand. But in case you choose to start with two(2) Legendary Creature/Planeswalker by playing two (2) cards (the cards with any ability to let you do so) in command zone, the starting hand size changes as following:
If a player uses two(2) commanders then he or she draws seven (7) cards and puts X cards in the bottom of their library in any order, where X is 1 plus the number of mulligans taken.
This way if players start the game with two (2) commanders and choose to take no mulligans, X will be one (1) and the player will have to put a single card from their hand on the bottom of their library. If they choose to mulligan once the number of cards put on the bottom of their library will be two (2) and so on in the same way for more mulligans.