Plagiarism is "not referencing your sources and (thus) pretending that someone else's ideas are your own". It doesn't matter how many or how few sources you used. If you don't reference them, you're a plagiarist.
Now, if you write that the French Revolution began in 1789, that Einstein is responsible for formulating the General Theory of Relativity or that the sun is ninety-three million miles from the Earth, then you're okay: these are established facts. Nobody is going to think that you are claiming you discovered these facts while the rest of the world waited on the edge of their seats.
If you borrow thoughts, ideas, techniques, opinions, commentaries, conclusions, strategies, designs, observations, research, criticisms, evaluations, judgements, anecdotes or examples (or anything else in that vein) then please, if only out of common decency, reference your sources and give credit where it is due.