Rule 603.8

Some triggered abilities trigger when a game state (such as a player controlling no permanents of a particular card type) is true, rather than triggering when an event occurs. These abilities trigger as soon as the game state matches the condition. They’ll go onto the stack at the next available opportunity. These are called state triggers. (Note that state triggers aren’t the same as state-based actions.) A state-triggered ability doesn’t trigger again until the ability has resolved, has been countered, or has otherwise left the stack. Then, if the object with the ability is still in the same zone and the game state still matches its trigger condition, the ability will trigger again. Example: A permanent’s ability reads, “Whenever you have no cards in hand, draw a card.” If its controller plays the last card from their hand, the ability will trigger once and won’t trigger again until it has left the stack. If its controller casts a spell that reads “Discard your hand, then draw that many cards,” the ability will trigger during the spell’s resolution because the player’s hand was momentarily empty.

Find this number in the current Rules