Rule 505.4b

During either main phase, the active player may play one land card from his or her hand if the stack is empty, if the player has priority, and if he or she hasn’t played a land this turn (unless an effect states the player may play additional lands). This action doesn’t use the stack. Neither the land nor the action of playing the land is a spell or ability, so it can’t be countered, and players can’t respond to it with instants or activated abilities. (See rule 305, “Lands.”)

Find this number in the current Rules