There is simply no way to determine this in an objective way. You cannot go into a study saying "I need to have five research questions." This is because there are bigger research questions and smaller research questions. Two big research questions might be sufficient for a study while it might require eight small ones. There is no way to define the size of the question or how many questions will be needed.
For example, in answering another question of yours, I suggested a study of the connection between educational levels and quality of life. That is one research question that would probably be sufficient for an entire study. This is because it could be broken down into many smaller research questions. These might include questions about the correlation between educational levels and divorce or drug use or criminal activity or reported levels of happiness. Several little questions like that are needed for the study, but only one question as big as the one I proposed.