Should amount of citation/references be taken into account when grading an assignment?
Plagiarism is not about whether your work passes some % threshold on software that detects plagiarism.
Plagiarism is the representation of words and knowledge/ideas as your own. When you learn from a source, you need to cite that source when you write about it.
Over time, some knowledge becomes so commonly known that it doesn't need to be cited any more in most cases; for example, Newton's laws. Lecture content might sometimes fall into this category and sometimes not, you can ask your instructor for guidance on their expectations. However, most of the content in a paper you write at a college level will come from a source you need to cite, unless it's work you've physically done or your own unsupported opinion.