This number is a rule of thumb for you to know that your topic is narrow enough that you can write a good paper. 150 results in a specific database, and 1,500 results in OneSearch.
Why do this? Consider someone who says they are going to write a 2-page paper on dinosaurs... I can tell you immediately, that paper would likely not be on a college-level, because they simply do not have enough space to cover the topic well. Due to the limited space, they would have to focus their paper significantly... for example, the discovery of Sue the T-Rex's fossil.
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