I know there exist approaches like EIDR (Entertainment Identifier Registry) and ISAN (International Standard Audiovisual Number). But I do not know how many movies are contained within those lists. Is there anything like IBAN just for movies?
This answer was posted as a comment by someone else. I'm just moving it to an answer, because I think it's the best answer possible.
IMDB has a unique ID number for each movie, tv show, and episode in its database. You can see it in the URL after the "tt".
This ID is used by various media apps (e.g. Plex) to download metadata for a video.
For example, the movie Avengers: Infinity War has this URL:
so it's ID would be 4154756.