In a Commander deck, also known as EDH (Elder Dragon Highlander), you are allowed to have a total of 99 cards excluding your Commander. Within these 99 cards, it is recommended to have around 37 to 38 lands. Lands are crucial in Commander decks as they provide the mana needed to cast your spells and abilities consistently throughout the game. Having a good balance of lands ensures that you have enough mana sources to play your cards effectively and not get mana-screwed.