I thought this might also be "it is not one person" but that would probably be more accurately translated as "nó không phải chỉ một con người thôi" I think? Because "it is not one person" has an implied 'just/only' which can be conveyed with intonation.
It implies that this "thing" (as "it") is not human, so could be an animal or monster or any non-human form. Saying "nó không phải chỉ một con người thôi" would mean that "it is not just a human" - but something more than that/beyond humanity.
Ok, so, lets assume I was trying to say. "it is just one person" as in "it is just one person, how dangerous could he/she be?"
or to use the negative as above. you see somebody ominously approaching you and as they get closer you realize that what looked like one person is actually a few people and you say to the person next to you "It is not just one person (but five)"