Essential Question

Why is it for project based learning, that there can only be one essential question?  I know that I need to make my curriculum relate to students to have higher engagement, grades, and motivation, but I feel that if I have just one essential question I may be missing out on standards and benchmarks. For example, last night I posted an essential question for Speak, which was directed to be student related. After thinking about the question over night, I fear that my question lacked depth. I also want to teach literary terms, plot, etc. with Speak but how do I do that when my essential question is solely based on a real life connection? Can I have more than one essential question or should I make the more English curriculum driving questions and mini lessons? I am just trying to figure out what how to incorporate everything I want to teach with Speak in one question...