He works in an office on the seventh floor.
Would "in an office" here refer to a set of rooms/offices, just one specific office/room, in which he works, or could mean either?
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It could mean either. My "office" is actually a desk in a room shared with 10 other people. Some "offices" are cubicles or have low separators. Moreover "working in an office" is often describing the type of work (filing, document preparation, finance, HR etc) rather than literally the location of the work.
So while it is quite possible that he has his own room on the seventh floor, and this is perhaps the most likely interpretation. It could mean he has a cubicle or a desk in a shared office.
It could mean either an individual room/area, or a set of rooms/offices.
Based on the example sentence alone, there is no evidence that the speaker more likely intends to talk about either concept over the other, at least in Standard American English. My intuition suggests that UK English speakers might be more likely to choose "office" to mean "one room" and "offices" to mean a group or more general place of business.
Sometimes offices is used, which would probably more likely denote the larger sense (a set or general area, floor, entire building, etc.), although the singular office is commonly used to refer to the same idea.