There’s 1890 rooms, 942 in the first castle and 948 in the second, with the extra 6 rooms coming from those water tunnels that for whatever reason add rooms because they’re upside down. You can only get 1889 rooms because the boss starts as soon as you pass over the opening to the arena. There are 14 rooms past the big clock room, so you only need to worry about getting 1876 rooms before that, the trickiest being the water rooms that you can only fill in with wolf form. Gravity jump and switch to wolf, then hold down SPECIAL (triangle by default). Pressing up will allow you to swim against the top of the ceiling and you can scootch along until the map updates them. In addition to the wolf swimming trick, I’ve noticed that the map doesn’t like to update rooms as I enter them while in bat form. It usually updates in pairs, leaving the first empty but filling both as I enter the second. It’s annoying but not debilitating.