So, 'library volume' can mean a few different things depending on where you're coming from. If we're talking about those big, honking hardcover omnibuses from a real library, they might collect three, four, even five novels from a series into one physical book. I've seen some sci-fi classics bound together like that. It's a space-saving measure for the shelves, I guess, but they weigh a ton.
Then you have the publishing side of things. Some 'complete' or 'definitive' editions sold as a single volume might compile a trilogy. Think of those fancy leather-bound Tolkiens. But honestly, the most common 'library volume' I encounter is just the standard, singular book as it was originally published. A library's copy of 'The Hobbit' is one volume. Their copy of 'The Count of Monte Cristo,' even if it's 1000 pages, is still one volume. The phrase is more about the physical unit the library catalogs than a specific count of narratives.
Online serials have messed with this, too. A 'volume' on RoyalRoad or a similar site might be 50-100 chapters of an ongoing story, later compiled into an ebook. That's one 'volume' containing what would be a novel's worth of content. So the number is almost always one contained story or story-arc, unless explicitly labeled as an omnibus.