Without a clarifying pronoun, necesita could be he, she, it or you need. So I suppose it depends on who you are talking to. So ¿Necesita una ambulancia? to the little old lady collapsed on the sidewalk is: "Do you need an ambulance?" To the parent of the boy who's fallen from a tree, it's: "Does he need an ambulance?" This is just a theory, though. I'll see if DL accepts "Does he need an ambulance?" next time around. :-)
Note that the ambiguity will always (I think) be dissipated by context: if you mean "¿necesita usted una ambulancia" you are making a direct question to a person. To use it in the sense of "¿necesita él una ambulancia?" you are talking about someone else, and that should be clear by context.