Rule 701.46c

If a player is instructed to venture into the dungeon while their venture marker is in the bottommost room of a dungeon card, they remove that dungeon card from the game. Doing so causes the player to complete that dungeon (see rule 309.7). They then choose an appropriate dungeon card they own from outside the game, put it into the command zone, and put their venture marker on the topmost room of that dungeon.

Find this number in the current Rules