My concern (Score 1)

I'm a student, I write my own papers from start to finish. If I'm writing about a popular topic it stands to reason that some of my content may resemble someone else's. Let's say one sentence out of 50 matches a sentence in another paper. Is this enough to be counted as plagiarism? I would say no. on the other hand one in 5 strikes me as likely being plagiarized. Where between the 2 points is the line between plagiarism and coincidence?