A score higher than 10% is troublesome. You want to aim for 5%.
Of course, the severity of the offence will vary depending on the context, but it is always best to err on the side of caution. If you are ever in doubt about whether or not something counts as plagiarism, it is always best to consult with your instructor or a trusted academic advisor. By taking these precautions, you can help to ensure that your work is original and authentic.