Numbers in Japanese are weird, and have multiple readings. There’s a native Japanese system ("koko" for 9) and a more common Chinese-derived system ("kyuu" for 9), but the number 9 actually has two Chinese-derived readings (the second one being "ku").

Different readings are used in different contexts. "kyuu no [thing]" is always a valid way to say 9 of something, but "ku" is used with some counting words and there are plenty of old-fashioned words and phrases using the native reading ("koko-no-tsu" is a very common way to say "9 [things]" or "9 [years old]").

The Japanese title is 極限脱出 9時間9人9の扉, with the subtitle pronounced "kujikan kunin kyuu no tobira". That’s really the only natural way to write it, so you don’t notice anything weird, but it’s definitely a choice.

The 「の」 particle basically turns the preceeding noun into an adjective, and nouns can be either plural or singular based on context. Taking those together 「9の扉」(kyuu no tobira) means "9 doors", but it can also mean "the 9 door". "The kyuu door."

In contrast, 9時間 (kujikan) and 9人 (kunin) are compound words that unambiguously mean "9 hours" and "9 people".