What is the optimal number of sources to include in a research paper, and how can one determine when the number of sources becomes excessive?

The optimal number of sources for a research paper varies depending on the topic and depth of research, but generally, including 5-10 high-quality sources is recommended. The number of sources becomes excessive when they start repeating information or when they distract from the main argument of the paper. It's important to focus on quality over quantity and ensure that each source adds value to the research.