Someone told me that there is only one magnetic field line in the entire universe. Has anyone else heard this? It doesn't make sense to me. Field lines aren't physical and just because there are no sources or sinks for magnetic fields doesn't mean you can't have a bunch of loops all over the place.
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There are arbitrarily as many magnetic field lines as you like in the universe. You could consider the north pole of any magnet, or electromagnet, to be a source, and the south pole to be a sink - or the other way around.
They are a human creation, often just used as a visualisation of the direction of the force that a test (test meaning exerting no force by its own magnetic field - a fictional object) north-seeking pole would experience at a particular point in a magnetic field; for instance, if I was looking at a bar magnet, I might choose to describe its magnetic field with 3, 5, 6, or however so many field lines I chose to draw around it from pole to pole.
