The first thing that comes to mind when you say design is usually the last output: bright screenshots, whereas those screens are the visible part of the iceberg.
The actual process proceeds through stakeholder interviews, user research, information architecture, wireframes, prototypes, and usability tests. Behind each screen are dozens of decisions and tried alternatives.
So as much as the question 'how many screens to design', the question 'what problems are we solving' is part of pricing and calendar. A nice screen that doesn't solve the right problem is a failed design.