In the
errata here. Yeah each remake expansion adds two more (closed) doors you can use.
Classic Game System: 16 closed doors, 5 open doors. In the classic they are removable cardboard so you can easy transform a "closed" door into open by removing the cardboard (whether or not you insert your own homebrew printed tile to simulate an open door or something else). The "extra" doors included in the expansions are just these cardboard tiles but still use the original door bases, so you are "limited" to 21 bases unless you 3d print more or scour the internet for individual pieces (don't do what I did!).
Remake edition they are solid plastic so that's a downside for simulating the above, but the upside as pointed out is that the expansions contain now bits of plastic to stand in as doors (even though closed, the "entrance/exit doors" don't have a separate open equivalent).
Remake: GS: [16 (open) + 5 (closed)] + [2 KK] + [2 ROTWL] + [2 FH] + [2 MOTM, when released] so using every single one, you could simulate the excesses of the Frozen Horror quests without buying duplicates, again if you really wanted to not physically re-use anything in a session.